The sexual function and role in development of testicles in animals has been known for millennia. There is evidence of castration, specifically gelding horses, from Scythian burial mounds c. 240 BCE. Similarly, castration of chickens (caponization) was practiced by 1555 and probably earlier.

The Greeks believed they were weights used to pull semen down from your brain (where they believed it was made).

See:

Aristotle On the Generation of Animals

"... since the testes are no integral part of the passages: they are merely attached thereto, just like the stone weights which women hang on their looms when they are weaving. When the testes are removed, the passages are drawn up within; this is why castrated animals cannot generate, whereas if the passages were not so drawn up they would be able to do so." "... All animals when castrated change over to the female state, and as their sinew strength is slackened at its source they emit a voice similar to that of females. This slackening may be illustrated in the following way. It is as though you were to stretch a cord and make it taut by hanging some weight on to it, just as women do who weave at the loom; they stretch the warp by hanging stone weights on to it. This is the way in which the testes are attached to the seminal passages, which in their turn are attached to the blood-vessel which has its starting-point at the heart near the part which sets the voice in movement."

And

Pseudo Galen in Definitiones Medicae "The semen, as Plato and Diocles opine, is discharged by the brain and the spinal marrow, while Praxagoras and Democritus and thereafter Hippocrates maintain it comes from the whole body"