Welcome to The Overworld, Nether Mobs!

Well Pigmens or the other Nether mobs might be bored, being stuck in the Nether forever. Why not let them explore The Overworld and get killed by players, possibly? By several nether portals appearing temporarily after two in-game nights pass! After the mobs get in The Overworld, the portal closes! Oh, plus let the small portals spawn near the players. :

Only these mobs can go:
+Pigmen
+Magma Cubes
+Wither Skeleton
-When living in a cold place, and a Wither
Skeleton or a Magma Cube spawns, they get hurt
because of the temperature, IT'S COLD!