A Horse That Is Being Ridden/Not Being Ridden By The Player Can Not Go Through A Nether Portal.

For The Horse To Be Able To Go Through The Portal While It Is Being Ridden And When Not
That The Horse Could Go Through A Nether Portal In Any Which Way.

The Horse Could Not Go Through The Nether Portal In Any Way.
The Horse Will Sit There In The Portal If You Are Riding It, Pushing It, Or Dragging It By A Lead

Steps to Reproduce:
1. Spawn A Horse.
2. Saddle It And Get On It.
3. Try To Go Through A Nether Portal While Riding It.
4. Try To Push It Into The Portal.
5. Try To Drag It By A Lead Through The Portal

"All Gamemodes"
I Added Some Screenshots To Give A Picture Of The Bug