I managed to get a Magma Cube out of the Nether.. don't ask how.. My own portal was to close to spawn, so I used the one of my neighbour. The cube followed me through the portal while mobs like Ghasts and Wither skeletons seem impossible. I found out that a Magma Cube can swim, so it followed me over water for about 100 blocks to end up in a nice cage behind my netherportal.
A shulker would be one of the most awesome mobs to have in the overworld, but we don't have access to an endportal to make some adjustments.
When 1.13 comes out i'm planning to make a recipe for zombiehorses to use in my own ssp world, instead of giving myself creative rights for a second. So I can spawn those horses which are ingame for years now..