Horses and nether portals not working properly

Loaded a 1.6.1 world with a horse stucked in the nether and could bring him back to the overworld. But then i couldnt bring him again to the nether(just for testing)
Then I took another horse to the nether and couldnt bring him back to the overwold.
Lastly I spawned with a spawner egg a horse n the nether and could bring him to the overworld, but I couldnt bring him to the Nether again.