You got sex with dogs and horses , you got bulls and pigmem , wolves , tigers , monkeys , insects , crocodiles , bears and so on ... what the else do you need ?! Its a fucking zoo all ready ! In the farm you got a horse and a dog (with several scenes each) , what else do you want ?!
I personally play the game mainly for the Ranger and Breeder Questline, so it's not too weird I'd welcome more content. Also don't put those two together, they are different Questlines - the Ranger one even featuring quite serviceable writing for a porn game.
The Breeder/Farm Questline in general is a bit underwhelming and feels a bit tacked on, there isn't much actual story going on with it. And as you say, it's Horse and Bull which is nice, but Dog? The dog doesn't work that well in the scenario in my opinion, having a domestic pig or sheep or goat there would tickle my fancy more and that'd be the reply to your question.
It's simply preference. I play the game not to collect all scenes but to experiences the content I think is kinky. And yes, having a different animal makes the whole thing feel different and special in it's own way, it's not "if you had one you had them all" kind of deal with a lot of other motifs you see in those games.
But that's a niche interest and that's okay!
EDIT: To add, also obvious cross-content between Farm and Ranger Questlines. Unlocking special features when you complete one in the other. Further up I wished for the completion of storylines having some kind of impact on the others or the sandbox gameplay, but each one feels very isolated.
