Karoo, also spelled “Carue”, is Vivi’s pet spot-billed duck, a fast-footed riding bird (one of the fastest modes of transportation available in Arabasta). He is the leader of the Super Spot-Billed Duck Squad in Arabasta.

Karoo is very loyal to Vivi as well as to her friends, the Straw Hat Pirates. He acts very unique as he does sometimes exactly what he is told, like giving his hand, or doing exactly something opposite like drinking all his water although Vivi tells him to conserve his water in the desert.

His role during Vivi’s time with the Straw Hat Pirates is mostly that of comic relief, although he does relay valuable information to Chopper from time to time, helps Luffy and Usopp to defeat the Baroque Works agents in Little Garden and saves Vivi from Mr. 2 Bon Kurei in Arabasta. He is the center of several jokes in the Arabasta arc, at one point being used as bait for fishing by Luffy and Usopp to Vivi’s anger and chagrin, and he is mistaken for an ostrich by Sanji.

(Source: One Piece Wiki)