Do horses have gigantism?

Clinical gigantism (acromegaly) is not really recorded in horses, although it occurs rarely in dogs and cats.

Extremely large heavy horses may be "giants" in a sense, especially as they sometimes have bone and metabolic issues, but I know of no case of true "giantism" in a horse. It might be that pituitary disorders in foals are so severe as to make the foal non-viable.