Quote from MrFiregem
When a horse spawns, all of it's stats are randomized, as is the case with almost every other Mo' Creature Mod mobs.
Quote from MineCrak
Thankfully breeding horses creates a chance that the offspring may have some better attributes than it's parents. So you could conduct a long-term breeding program to create Uber-Horses!.
Quote from Skylord1
Yeah Tier Horses I hope for more special bred for horses that have good stats. I dislike flying horses though, kinda OP once you've got it, u'll never walk again D: