I was herding cows on a survival server and one of them accidentally went into my nether portal. I tried to get it back, but it would simply not make the return trip through the portal as I would have expected. Instead, it simply walked through the portal like it was an air block and failed to return to the overworld.

So I went to a creative world and led cows through a nether portal. Sure enough, they went through the portal. Then, I tried to bring them back. Again, they walked right through the portal and failed to return to the overworld. Then, I spawned some cows in the nether to see if they would go through the portal. They did, so I tried to bring them back to the nether. They failed to make the return trip to the nether.