The Cerberus is a Legendary pet in Bubble Gum Simulator. It can be obtained by opening an Ancient Egg. The chance of hatching it is 1 in 33,333 (0.003%), or 1 in 16,666 (0.006%) with the Lucky Chancesgamepass.
It could also have been obtained in the Season 2 Bubble Pass Rewards upon reaching 1,150 points.
It could also have been obtained by claiming the tenth slot from the Season 3 Premium Bubble Pass rewards upon reaching 700 points.