Title 

Tried on multiple worlds, letting them walk in, nor pushing them in works.

 I am aware that mobs have a cooldown between when they can enter portals, but the cat shown in the picture and the clip has never been in the nether.

Edit: Started working suddenly, only closed and reopened the world, have no idea what could be causing this.
Edit 2: It seems very inconsistent on when it works or not, I got it to work once on my singleplayer world only after closing, waiting a bit, and then reloading it.