Theory:

All the animals in Bojack Horseman became bipedal fairly recently due to some kind of science dodad. Like maybe 50 years ago. Give or take 25 years for them to comprehend the english language. This is why animal people and human shows were so popular in Bojacks prime and why a lot of the domesticated animal characters have pet names like Mr. Peanut Butter or Princess Carolyn. It also explains why they haven’t evolved beyond their animalistic reflexes yet.