Boss

Boss (ボス Bosu) is an unique large beetle from the Little East Blue Arc. He was originally a creation of Dr. Indigo, but escaped.

Abilities and Biology
Unlike regular beetles, Boss 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 a runaway from Shiki the Golden Lion, who had been creating an army of super animals to take over the world.

History
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 Corto, 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 molted into a new form 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. Freed from the net by Zoro, Boss is able to regain some of his power, which he use to defeat Corto, first, and then to rescue Luffy and Brook who were falling in to the sea after the battle with Largo. He and the Straw Hats' captain eventually have their match, but it ends in a draw since none of them wants to give up. In the end, Boss stays with Yoko and the other inhabitants of Little East Blue, continuing his work as their protector. Though promise Luffy as he leaving they'll continue their fight again someday.

Trivia

 * He has the Marine logo painted on his right side, something Yoko gave him.