Generally, "mutants" don't breed "mutants". Foe instance, I have a 6'7", 300-lb. friend whose parents are normal stature. His size is not due to a medical malfunction, & his own grown children are normal stature. And I have a Siberian Husky that's purebred, but is twice the size of an "average" Husky & weighs 120 LB. His littermates are "normal"-size, as are his "parents", & the one litter of pups he's sired. Other than his size, he has all the traits, actions, & tools of any other Sibe.
If the "eohippus", the first known horse species, the size of a Collie, was around today, it could breed with a modern horse & produce young, if physically possible. Eohippus didn't "evolve"; it slowly became larger thru successive generations, as its environment became more-favorable for horses. A main difference between eo & today's horse besides size is that eo has a toe on each side of each hoof, which reached the ground. I'm guessing these toes helped eo to often outrun its would-be predators.
If the "eohippus", the first known horse species, the size of a Collie, was around today, it could breed with a modern horse & produce young, if physically possible. Eohippus didn't "evolve"; it slowly became larger thru successive generations, as its environment became more-favorable for horses. A main difference between eo & today's horse besides size is that eo has a toe on each side of each hoof, which reached the ground. I'm guessing these toes helped eo to often outrun its would-be predators.