Ok so once you get fucked by the wolf man (or the horse man he's in the stables to the right of the city gate) you will start to get pregnant over time. Once you're full term, go back to the side of the Church where you first spawned in at the beginning of the game. There's a set of spots where you can both Rest (this resets all game events) and Give Birth (you do this when you are big enough and it creates a baby).
Once you have done that, go to the cowgirl and drink milk in order to build up your own milk supply (its in your tits). The catgirl needs to do this twice in order to get enough, so you will have to sleep at spawn in order to reset cowgirl to drink a second time.
Once you've done that, you can go back to spawn and nurse your baby (walk up to them and press the icon, like you did with sleeping and giving birth), this is an animation that takes a little while. Once your baby drinks enough milk, two icons will be displayed; PICK THE FOREST ICON, that lets you play as the creature you just gave birth to. The nest icon will cause the baby to disappear and you will keep playing as the catgirl.