We hear this a lot; that ghasts (bunny-ears) "sometimes" spawn near portals in the "real world"; but I can't for the life of me remember having seen any actual evidence of this being true…
Do ghasts actually spawn near portals, or is this another Herobrine?
From the Minecraft Wiki
Notch added a small chance for Ghasts to spawn in the Overworld near Portals in Beta 1.5. A Ghast spawned outside of the Nether will fly everywhere, mainly above the clouds. Can even spawn extremely rarely without a portal.
This is my understanding of the situation:
I believe the latter has been found to be the case by those who have studied the code of Minecraft, but I don't have a citation handy. Minecraft Forums would be the place to look for one.
It can spawn if the portal is at least 4x4 blocks high. While extremely rare, they can also spawn without a portal.
they can't spawn in overworld, but they can get to overworld through portals if you built it 4x4 or bigger!!! .. making a security caution of building 4x4 nether portals so they can destroy your wooden house if they attacked you inside it.