Zombie Pig should spawn in the nether portals , but when it happens they disappear instantly. This did not happen until the 0.12.1 Build 7.
I tested the Windows 10 edition and Android 5.0 and both occur this error.
Update by nighter
This bug has been reported to our internal bug tracker for further testing and a fix. It is scheduled to be fixed in one of the future updates (no specific date can be provided).
Please avoid duplicate comments. Post only NEW information regarding this bug.