/summon EntityHorse {Type:1-3} always summons a horse, never a donkey, mule, zombiehorse, etc.

Playing in survival mode with cheats turned on:
/summon EntityHorse

{Type:1}

command ALWAYS produces a horse. Different colors, sometimes even babies, but never the donkeys I expect.

{Type:2}

also gives me a horse, not a mule.

{Type:0}

unsurprisingly also gives me a horse, but that's expected..