The Groo is the main antagonist in the YoshiEgg (series). His species was very thriving in ancient times, but after trying to wipe out the Tanooki species, the two species had a brutal war, leaving only one Groo left. Since the ancient times, the Groo grew more intelligent and evolved, making him one of the most violent, ferocious, and smart animals in the world. The Groo has grown a strong hatred towards all Tanookis, especially YoshiEgg Nook, but no one is sure why. It says in the handbook for The Adventures of YoshiEgg that The Groo and YoshiEgg's great, great, great, great, great grandfather fought in the ancient times, but that has not been proven true yet.
The Groo appears as a secret Eidolon in the game. You must enter a secret room in the Dwarven Kingdom and he will attack you, and after you beat him you can summon him.