Do all animals have a clitoris or is that just humans?
There are other animals that do have clits, examples of this would gorillas and pigs.
It has an evolutionary purpose to help sex feel good so we will continue doing it; furthering the species.
(not every species has the consideration to put a little spit of it if it doesn't go easy so a sense button was made that would trigger arousal when pushed, like the clitoris)
But not all animals have this. There are many types of reproduction, like that of Salmons that happens outside of the body. Therefore these animals would not require a center of stimulation like a clit.
They do however becomes stimulated but it is through their picking up of a scent put out by the other sex of the species.
Or like in cats where the male will have barbs on his penis that will stimilate the movement of eggs down into the into the uteris, but it would not be the same style of sexual stimulation that could be compared to human stimulation.