They also left out buffalo, parrots, cats, donkeys, sentient cows… really it is because the writers essentially rebooted the creature makeup of Equestria as they went along.

@Background Pony #C8BA
Mane6 had no problems visiting yaks, dragons, griffons, insects and dildo-on-forehead equines

Its because zebras come from very very far away. It's super rare to see a zeeber in equestria.

because fuck you worthless barcode on legs, littered with junk and inferior genes