First of all, can we take a minute to realize how two horse legs and horse body block make the perfect butt cheeks?
Anyway. When you're playing in Adventure Mode, you take what you can get. If you kill a chicken and a dog and break open an Octocrab egg, you will have to make do with the parts you are given. Unlike creative mode, you most likely won't be able to build a perfectly symmetrical creature. Body blocks may have to be used for heads. You might have three legs instead of four. But that's ok! I actually think they look awesome, like a rag-tag team of misfits:
Yesterday Roman made Horseman, a super fast, one eyed, six beaked creature with dog legs for arms (making him look humanoid). Having six beaks made him really powerful, and his horse legs made him fast! Those chickens were no match for him. However, when Horseman annihilated his first victim, he fell to his knees in grief. I don't think Horseman knew his own strength!