He is a brown dog, with floppy ears and a red studded collar.
He seems to be a free spirited animal, as he tore off the arm of a stuffed citizen of Dressrosa and ran off with it.
Mario was seen stealing an arm from a living toy citizen in Dressrosa. He ran off with it as the man tried to keep his stuffing from falling out as he ran after the dog.
Manga and Anime Differences
In the anime, just after Sugar had been rendered unconscious and her Hobi Hobi no Mi powers deactivated, Mario was finally caught by the toy man, who grabbed his arm and reattached it while turning back into a human, causing Mario to look at him with overwhelming shock and run away in a panic.