While this game technically has bestiality it is not a developed theme. There is a facility upgrade in the mansion for the Kennel, which unlocks the ability to add dogs and horses to the sex interactions. However, they are just animals in name and have no explicit bestiality mechanics, so you are better off with Beastkin slaves.
As far as related mods go, the Bugfix mod fixes the races of the dogs and horses to at least be close, and AricsExpansion adds minor bestiality mechanics to the Kennel during the end of day as well as impregnation by animal.
If you want more bestiality, then you would likely need to make your own mod or make something to add to AricsExpansion.
