First of all, whether animals have emotions (or possess emotional intelligence) is an ongoing debate across scientific disciplines. Biologists, psychologists, and philosophers can all add to this debate, so if you want to know more about both sides of that argument, your question might be better suited to other scientific subreddits. Mark Bekoff has written a beautiful multidisciplinary overview that includes how giants in their respective fields, like Skinner and Panksepp, view emotions. I will link his article below. It is not a historian's task to define emotion, but the idea that animals do not have real emotions is not some ancient misguidance, since some scholars adhere to this position in the 20th and 21st centuries.

EDIT: u/pensadesspo*'s answer goes into more detail in regards to the philosophical issues at hand. Don't miss out on that contribution and scroll on after reading!*

Secondly, I would advise against labelling a position in this debate as 'stupid'. Psychologists have enough difficulty defining what emotions are exactly, even in humans. What you actually mean by emotions and whether animals actually possess them is by no means a simple matter. Rather than asking why and for how long people were stupid, one might ask how smart people came to a conclusion that sounds strange to you. Perhaps they can persuade you, or perhaps you can rebut their claims with a more informed opinion. So, did these authors conclude that animals had no emotions, and why would they conclude this?

As far as I am aware, Thomas Aquinas never claimed that animals lack emotions. On the contrary, Aquinas sees emotions as a quality that humans and animals share. In the Summa Theologica, a chapter is even named "Of the Passions, which are Acts Common toMan and other Animals". The key difference between man and animal, in his opinion, is rationality. On the other hand, some rational thought is required to experience emotions - I need to understand that a lion has the potential to kill me before I can be afraid of it. Since this is slightly outside of my comfort zone as a historian, I will quote R.C. Roberts, professor emeritus of Ethics, to illuminate Aquinas' views: "Aquinas holds that humans have emotions in common with the non-rational animals. If emotions have a logic (and not just a causal story), how can he hold that non-rational animals experience them? Aquinas answers that "intrinsically considered, the emotions are common to men and the other animals; but in so far as they are under rational control (a ratione imperantur), they are distinctively human." Animals and humans alike experience emotion, but a human's greater capacity to reason allows him/her to control these emotions far better, and act upon them in different ways. To return to the main subject - Thomas Aquinas did not think animals lack emotions. He did have little compassion for animals, especially compared to humans. Emotions had little to do with that position, but rationality decided the difference between humans and other animals for Aquinas.

On to Descartes, who saw animals as machines. In his Discourse on the Method, he wonders whether "machines bearing the image of our bodies, and capable of imitating our actions as far as it is morally possible" could ever be distinguished from real humans. The answer is two-fold. First, humans can express complicated thought through language. Second, such a machine will not act based on reason or knowledge, but "solely from the disposition of their organs". Descartes observed both animals and humans, noticed that animals could not express complicated thought and mostly seemed to act based on primal needs like hunger and reproduction. As far as I know, Descartes does not state specifically that animals lack emotion, but I can certainly see why you place his comparison of animals with machines in that tradition. Again, the discussion revolves around rationality, rather than emotion.

In conclusion, I do not believe either of these men truly believed animals completely lacked emotions. Both saw reason and logical thought as separator between man and animal, which lead them to see humans as superior. If you think humans are superior to animals in every way, the idea that animals' lives are worth less - or worthless - is not too far-fetched. This view might be incongruous with (parts of) modern society, but that is another topic entirely.

I have tried to illuminate the views of the people you mentioned, but I believe you could learn a lot more from asking other scientists about the nature of emotion and animals. What are emotions, do animals have them and what separates mankind from animals (if there is such a separation at all)? Learning what our ancesters thought about that is only the first step in that particular forest of knowledge.

Sources:

Thomas Aquinas, Summa Theologica, Vol. II.

René Descartes, "Discourse on the Method," Part 5.

Robert C. Roberts, "Thomas Aquinas on the Morality of Emotions",History of Philosophy Quarterly 9, No. 3: 293.

Marc Bekoff, "Animal Emotions: Exploring Passionate Natures." BioScience. 50. 861-870.

Floyd, Shawn D. “Aquinas on Emotion: A Response to Some Recent Interpretations.” History of Philosophy Quarterly 15, no. 2: 161–75.

First, this is certainly one of misintepretations-or maybe, misconceptions-against Descartes. Second, Aquinas never wrote that it is not a sin to harm animals 'because they do not have emotions'. And whilst the logic of Aquinas does share a portion with that of Descartes, it widely differs from who draw what from which, for while both were heavily religious, Descartes is called 'Rationalist'; and Aquinas, a saint, and a 'Theologist'. Finally, the trend of thoughts in philosophical history and who influenced whom is of such a vague question with following vague answers, unless the person in manner quotes or mentions another, or such paramount evidence shows he is sided with some set of ideas predecessing him.

Beforehand, a simple example. Consider that you see a cat in front of you, trying to catch a mouse. It may adopt a 'strategy'; it may cry out when the mouse bites, or growls when things do not happen as it 'wished'. Conversly, the mouse would seem 'terrified'; it trembles and has its eyes wide; it squeaks everytime a paw or teeth passes it by a inch.

Question: do they feel emotions? Undoubtedly, most would answer, yes.

Another question: it turns out that this cat and this mouse was a carefully articulated machine with bones out of metal and blood out of electricity, by the most prominent scientists. Does these machines have emotions? Would you still answer, yes? If not, why?

'Animals have nerves and brains, they are 'Organisms''; but before you have opened up the body of the cat and mouse, can you tell which is a 'organism' or, merely, a 'machine'? Does it really make a difference to have bone of calcium rather than of metal, and blood of complex proteins than flow of electrons?

Or is it, as Thomas Hobbes puts it: 'For seeing life is but a motion of limbs, the beginning whereof is in some principal part within, why may we not say that all automata... have an artificial life? For what is the heart, but a spring; and the nerves, but so many strings; and the joints, but so many wheels, giving motion to the whole body, such as was intended by the Artificer?' - Leviathan, Introduction

Taken all this, what is it to feel; to have emotion? Is it truly the same thing that is in matter when we scream in pain, and a dog apparently screams in pain?

Sorry for the long introduction, but the question of 'feeling' is much more complicated than we normally believe to be; and to say Descartes as dumb-da-dill donkey is to ignore the essence of such questions. With this in mind, let's examine Descartes first. His most famous sentence 'I think therefore I am' or 'Cogito Ergo Sum', is the core factor for understanding his intepretation upon animals. To summarize his ideas:

1. I am, at least, thinking properly.

2. Yet, my senses can have errors; everything I perceive might be deceived.

3. After reducing everything that I cannot assure to be unerronous, and false, all sensations and ideas fade away, remaining only the mind itself.

4. Therefore, the essence of my being is not material; but it is the thinking me, the one that makes the verb cogitas to cogito: the rational soul.

5. Further, I have the idea of God, the perfection in every way that I am defect of; and certainly I cannot derive myself independently, for if I was the sole reason of my existance, I would not feel any defects in myself. Therefore, God exists; and whilst his infiniteness assures his self-existance, I am surely his creation, owing my existance and all rational faculties to him.

6. That my essence is solely determined by the fact that I am a thinking being is thus evident. But the sensations and perceptions which is surely not of immortal, indivisible soul's aspect, alongside with a clear idea of my bodily existance, makes me believe there does exist a body and an external world; and since God is not deceitful, it is most certain that my body exist, and the external world also.

7. Whilst body and soul is of such difference, they interact, and the bodily sensations sometimes create errors; and the thinking upon experience and free will also have errors; thus man's life are full of mischief; yet still, it is no mean for the finite to blame the creator for his imperfection; rather, I thank God for everything he has blessed upon me.

This is called mind-body dualism; specifically, Cartesian dualism. And as you can see, the key point in human is not its machine-like body(Descartes specifically says that the natural body is no more than a machine articulated by God) but its rational soul. And while the two components are distinct, they are so closely united and casually interact with each other, as we see, hear, feel, enjoy, and weep: 'it is not sufficient for it(the soul) to be lodged in the human body like a pilot in his ship, except perhaps to move its members, but that it needs to be more closely joined and united with the body in order to have, in addition, feelings and appetites like the ones we have, and in this way compose a true man.'

Thus here is the core of problem: if we are to say animals feel as we do, we must assert that they have the soul as we do. He of course denies this for the following factors:

1. Language and sign usage in a complex manner is essential to humans and humans alone; a machine would never imitate above simple utterance according to programmed ways. Since animals have such many organs corresponding to us, if they had mental capabilities then they will be able to and would communicate with us as all rational souls will, they fail to do so; thus, their apparent 'communications' and 'emotions' are but machinary.

2. Whilst human reason is a universal tool upon any matter, animals show that their abilities, while some being much superior to humans, fail to show such universialness beyond its designated designs; the purpose of that particular machinary.

'Rather, it shows that they have no mental powers whatsoever, and that it is nature which acts in them, according to the disposition of their organs; just as we see that a clock consisting only of ropes and springs can count the hours and measure time more accurately than we can in spite of all our wisdom.'

Thus, he does not claim that there is no reaction which is seemingly an emotion felt by animals; for their bodies are just like our material bodies, a machine. Yet they lack the most important factor distinguishing human above all else; the rational, thinking soul: therefore, in this sense, they cannot feel.

I strongly suggest that you read the Discourse on Methods and Meditations on First Philosophy. All sources, except from Leviathan, is from these two books. Citations are from Discourses, part five.