What is the rarest Adopt Me pet?

I'm curious, what exactly is the rarest Adopt Me pet in the game? Is it a mythical creature that players can only dream of owning, or is it a more common animal that has an extremely low drop rate? I've heard rumors of exclusive pets that are only available through special events or promotions, but I'm not sure which one holds the title of the rarest. Could you enlighten me on this matter, and perhaps share some insights on how players can increase their chances of acquiring this elusive pet?