What I expected to happen was...:
A horse to spawn from the horse spawn egg and to stay spawned.

What actually happened was...:
It spawned and the game removed it less than a second after that.

Steps to Reproduce:
1. Use the Spawn Horse Egg
2. The horse vanishes

Initially I thought that horses are biome bound so I went around though different biomes and tried but the game just removes the horses immediately after they are spawned. I tried a whole set of things such as changing game difficulty, changing gameRule for spawning mobs and so on, but nothing really helped.

I cannot spawn a horse in no way what so ever and make it stay spawned.

Also, I should note that the world is old, created maybe in 1.4 and is a server world.