tutorial on how to do this

1. build 3 blocks up on one of the edges of the obsidian end platform
2.  place a rail 
3.  place around 30 tnt minecarts
4. get back to the overworld
5. get a wolf through the end portal and jump in without armor
6.  you'll die; then the wolf will die then respawn go back through
7.  and the wolf will be there
8. (btw I think it has to be tamed)