We can't say with any definitive authority who "the ancients" in total were in the mind of Aristotle however we do know some specific individuals that he considered to be part of that group and can draw some inferences from that information. As a matter of obvious formality, let me start by stating that Aristotle lived from 384-322 BCE so obviously his usage of the phrase "the ancients" will be skewed based on their distance in time from his own period.

Some of the individuals he uses that phrase for are actually not 'ancient' at all. For example, towards the end of Part XV of Book Three Aristotle mentions 'the ancients' and then immediately, in that frame of reference, names Dionysus the Elder, a statesman who lived 432-367 BCE. That's someone who lived around 15-20 years in Aristotle's own lifetime and he's still got 'the ancients' tagged on him.

On the other hand, in Part V and XIV of 'Politics', Aristotle goes the opposite direction by including Homer as one of 'the ancients'. Now we are all, I'm sure, aware of how difficult the Homeric Question is today but putting aside the single authorship question and trying to place ourselves in Aristotle's mind, we can assume (with reasonable doubt) that he would believe something in the ballpark of the running estimate of Homer's age at the time, placing Homer to have lived sometime around 1,000 BCE.

Other people that Aristotle seems to include as "ancients" are: Pittacus (640-568 BCE), the legendary Solon (638-558 BCE), and a few figures from Persian history.

It seems important to note here that when Aristotle refers to "the ancients", he seems to be invoking "ancestors" rather than "the Magi of the time-before-time" that the word "ancients" seems to imply. Some of the individuals he refers to by the term have only been dead (to him) for 150 years whereas others may have been gone up to 700 years at least. I keep trying to find a better word to translate the Greek with but I can only think of "predecessors". That's obviously not a great translation of the word for linguistic purposes but from a cultural-literary standpoint it provides ample context for what Aristotle means when he says 'the ancients', namely that its generally anyone who came before.

For further reading I'd recommend Dr. John Palmer's essay on "Aristotle and the Ancient Theologians" .