The Baby Grinch's Mischief Whistle is a pet summoning item which summons a pet Baby Grinch that follows the player around. Baby Grinch's Mischief Whistle has a 6.67*1/15 (6.67%) chance of being dropped by Ice Queen during the later waves of the Frost Moonevent. When the player is far enough away, a balloon will carry the Baby Grinch to catch up to the player.
This item is a reference to the story How the Grinch Stole Christmas! by Doctor Seuss and the live-action remake, where they included a scene of people having a flashback about the grinch as a baby.