The Nether Boss does not spawn in the Nether
Sorry if this isn't a bug
Linked issues
Comments 4
Thats whatim doing, and its not working. It shows the boss for a few seconds, then it disappears.
Change your difficulty up from peaceful.

Duplicate of MC-155?
Spawn naturally, or do you mean can't be created?
I ported to the nether in creative in 1.4.2 and successfully summoned a Wither Boss by placing the soulsand and the skulls.