Java Edition:Ender Dragon in Invalid Dimension
The Ender Dragon, which is not supposed to be able to go through any portal, may be brought to the Overworld with two methods.
Obtaining
Push Through Portal
To test: Test start version.
See video by Jellymeep. Works from 1.14[test] to 1.21.4 (present).
22w13oneBlockAtATime
In the April Fools' snapshot 22w13oneBlockAtATime, after destroying all the end crystals, the ender dragon can be ridden by the player. The ender dragon will follow the direction that the player points in. If the player directs the dragon downwards, it will enter the overworld at y = -32.
23w13a_or_b
In 23w13a_or_b by voting for "Replace all <ITEM> with <ITEM> in player inventories" it's possible to replace any item with Ender dragon spawn eggs. These can be used to summon ender dragons in the Nether or Overworld.
Also by voting for "Replace natural spawns of <ENTITY> with <ENTITY>" it's possible to replace the generation of mobs. This allows the player, for example, to generate ender dragons instead of skeletons at night.