Boss

Boss (ボス Bosu) is a very large beetle, probably unique. Unlike other beetles, he continues to grow and to change shell even as an adult, this causing his unusual giant dimensions. He lives in Little East Blue with Yoko, an orphan girl who found him in the forest some time after her father's death and got attached to the beetle seeing in him her lost dad. Since then, Boss protects the island from the pirates. He becomes stronger every time he grows up (once a month), making him a very powerful fighter, capable of emitting fire from his horn. He has an incredible stamina, which allowed him to resist to Franky's Coup de Vent and Luffy's Gomu Gomu no Bazooka without serious wounds, and Yoko said that he can heal quickly overnight underground. According to Amigo Pirates, he's been stolen from Shiki the Golden Lion, but the events are so far unknown.

Before finding out that Yoko was his owner, Luffy wanted to capture it and have it be their 10th member of his pirate crew. He later dropped this idea after seeing Yoko's determination, yet challenged Boss to a fight after he became stronger, something he agreed to. When he was preparing for the evolution, Little East Blue was attacked by the Amigo Pirates, who wanted to capture him as a gift for Shiki. After being defeated by Largo and Colt, Boss agreed to follow the pirates for saving the island, but was stopped by Luffy who wanted to continue their challenge. After being hit by Straw Hat, Boss evolved and attacked again the pirates, managing to free Luffy, Zoro and Sanji from their nets, but falling again under the attacks of Largo because his mutation has been too fast and his power was not completely restored.