Okay, it's offical
Sentinent Goats exist, and minotaurs are around three to four times the size of a pony, which puts them around Buffalo size. Fittingly.
EDIT: Yeah, Nido, sometimes it is best to leave a creature with a single main caste. Some creatures are just fine with a single caste, and even if they did have more than one what would it add, realy? A single, "lesser" civ that trades good stuff is kind of ruined when they have gigantic humanoid bulls randomly walking around. My opinon? Buff up the size of vannila minotaurs, add in Chimeras and Phenoxes (perhapes with some sort of Greater Cockatraice, if you have made one already?), and you have the basic semimegabeast's done.