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This data pack changes what happens when pets (tameable mods) reach 0 hit points, instead of dying the pet enters the dying state.
While in this state it:
How do you get your pet out of this state? By feeding it! Once your pet is at full HP it reverts back to normal!
Important: Anything that would bypass a totem of undying would bypass this feature (the void or /kill)
Pet list:

This data pack makes pets (tameable mobs) unable to die*.