Black Hayate

Black Hayate is a little puppy that was found by Kain Fuery on the street in a rainy day. However, Fuery couldn't take care of the puppy because he lives in a dormitory. He asked all the possible people that might take care of it: the Elric brothers, Jean Havoc, and so on. He didn't find anyone that could take care of Black Hayate, so, Riza Hawkeye adopted it. On a side note it has been speculated by fans that Black Hayate is in fact a Shiba Inu. In Chapter 87 of the manga, after Fuery mentions that Black Hayate's ability to sense Homunculi was incredibly useful and that he should be considered a ranked officer, Mustang gives him the unofficial rank of Second Lieutenant (to Fuery's dismay, as the dog now outranks him).

Anime
After his adoption, Black Hayate becomes very disciplined under Riza's care, having to sit, shake, and lay down before being allowed to eat. Black Hayate apparently likes to bury bones within the old execution grounds near Warehouse 12, as seen in episode 37.