In his normal form, Pierre is a large pink bird with a red polka dot pattern on his body. He's solid red on the sides of his head, the back of his head, and on his horn-like ears. He also has bandages wrapped around his torso. He has a tab face, with a yellow beak.
When in his hybrid bird-horse form, he transforms into a horse with wings that maintain the pink body with red polka dot design, but with white feathers. His face also elongates to that of a horse's. In a way, he is supposed to resemble a Pegasus.
As Gan Fall's pet and stead, he is very loyal to his master, going as far as to bite anyone who talks bad about him.
Pierre ate the Uma Uma no Mi, a Zoan-class Devil Fruit which allows him to turn into a full horse. This is supposed to allow Gan Fall to ride Pierre and travel in the skies, but as noted by the Straw Hat crew, since Pierre is already able to carry Gan Fall in his original giant bird form his power seems quite useless aside from the aesthetic effect. However, Pierre can use his powers as a horse offensively (albeit for comedic effects), by using his horse teeth to bite the head of anyone who's talking bad about his master in front of him.
Sky Island Saga
Pierre mainly served as transport for Gan Fall throughout the battle againstEnel, such as greeting the Straw Hat Pirates upon their arrival and carrying Conis away from a blast launched by Enel. Aisa also rode on Pierre after Gan Fall was forced to leave him behind to help her and Luffy escape from the giant snake Nola and later the two observed Luffy facing Enel before Pierre is knocked out when protecting Aisa from Enel's attack. Pierre later woke up at about the same time as the unconscious members of the Straw Hats just as Luffy finishes off Enel.