The lead designer of Minecraft, Jens “Jeb” Bergernstern, announced today that the upcoming version of the game will allow mobs (e.g. cows, chickens, creepers, the entire bestiary and array of monsters in the game) to move through Nether portals.
At present, only player characters can move through Nether portals, but the future patch will remove that restriction and allow every creature in the game to transition through them.
It’ll be possible for Ghast fireballs to blast back through the portal and damage your base, so it’ll be a good idea to keep the portals turned off when they’re not in use unless you really enjoy hearing screams and scary noises coming from the basement or whichever godforsaken place you have your portal to the Nether located.
That said, you could always set up a piston-powered shield in front of your portal to ensure that the portal closes whenever you step through it—kind of like the iris system in Stargate SG-1. How’s that for an idea?
It wouldn’t hurt to line the whole place up with obsidian, either.