My Mythic-Grade Pet Slime

Finn Lane is tested and found to have an 'E-Grade waste talent', the city's laughingstock. He picks up a dying Slime—only he sees its 'Mythic-Grade' tag. Everyone mocks him for raising a 'Jellybean', but the Slime devours an S-Grade Secret Realm lord. When enemies surround him, it transforms into an abyssal tyrant, devouring all. His mythic pet beast always tries to get him extra meals.