The pet terrapus egg is much rarer loot than the typical terrapus egg! When placed, it will have the normal egg characteristics, besides for one small difference. It's your pet! When you come near the egg, the pet terrapus will hatch, but instead of attacking you, it will run right through you. Make sure you have an enclosed area that the pet terrapus cannot get out of, though!
This item can be found in mechanical chests or traded in market worlds.