Mob issues with Nether Portal

Some entities after going through a portal will not exit any other portal in the nether even if portals are linked/created new.
And also some mobs (like Zombie Horse) will not enter the portal at all.

I have taken chest boats, dogs, cows, mooshrooms, donkeys and horses into the nether and traveled various distances before trying to leave via an already linked portal and the refuse to exit, whether pushed into portal, pulled via leads, even tried in minecarts or boats all with no success. Even the boat by itself wouldn’t go through!
After exiting, re-logging in. Breaking portal, creating a brand new portal far away, still nothing. They just refuse to leave the nether. And when TP command was used by an admin on one named animal, it disappeared but didn't reappear and was gone from the game.