Summary

Non-player entities that enter a Nether portal in the Nether do not re-appear in the Overworld until the server restarts.

Test case

1. Tame a horse.
2. Bring the horse into the nether.
3. Bring the horse back out of the nether.
4. Restart the server.

What happened

The horse was not visible until step 4.

What I expected

The entity would be visible by step 3.

How that differed from my expectations

The entity disappeared until a server restart.