Glow Squids and Axolotl apparently, will spawn in the End Dimension if there's water present and light level enough for them to spawn just like in the overworld. 

To reproduce the bug: Go to the End, place 2 block high water (or higher)(the width matters, the wider the water is, the more chances of them to spawn), cover the water with solid blocks for the light level required for them to spawn(you can use tinted glass for you to still see if they're spawning), go 20 blocks away from the water or the distance that mobs can spawn, axolotl and glow squids will now spawn, turn on peaceful mode to maximize their spawn rate and to get rid of hostile mobs in the End(enderman).