Java Edition:Bob's Dogs
In the April Fools' snapshot 20w14∞, it was possible to get tamed dogs whose owner UUID matches no player. The dogs are intended to be owned by the villager Bob but Bob has a different UUID.
Obtaining
In the Easter egg dimension isolation, at 0,0, a house generates with 3 wolves inside. The wolves are "Bob's dog", "Bob's Other Dog", "... their cousin Jim". The wolves have blue, green, and red collars respectively. The names and collars can be changed so they can be made to look identical. However, there is unmodifiable NBT on the wolves under Modifiers:[{Amount:}] that differs depending on the wolf and can be used to differentiate the three. For Bob's Dog, this NBT is set to 0.05350075638701333d, Bob's Other Dog is -0.015971064955500245d, and ... their cousin Jim is -0.02909160442903736d. The Owner UUID of all three dogs is [I; 1490639820, -1482404121, -1731621927, 1035587035] (f41a81cb-2958-4d80-918f-41151e02beb4) which does not belong to any registered Minecraft player. To keep the wolves past the update they must be sent to one of the normal dimensions. This can be done by creating a nether portal or an end portal using end portal frame items.