IRL Spe was a terrible stud because he hated mating with mares. When forced to, he'd get angry and bite his mates. Mejiro Dober being a notable exception, being one of the few horses that IRL Spe had taken a fondness to.

IRL Spe was generally also pretty antisocial around horses and had a hate boner for chestnut coat horses in particular. He was raised by a human caretaker (Tina Price, the RL inspiration for Sweek's adoptive mom in the anime) for a while after his mom passed, so he probably imprinted on humans.