Black Hayate is the dog that was adopted by Riza Hawkeye.
Originally a stray puppy in Central City that was found by Kain Fuery during the rain. He couldn't keep the dog since he lived in the military dorms. Lieutenant Riza Hawkeye demanded Fuery find him a home or put him back, but his coworkers couldn't keep the dog for various reasons. As the day was ending, Fuery couldn't find a home for the dog. Lieutenant Hawkeye arrived to check on him, but she took the puppy in when no one else would. She fed him, but he went to pee in the corner. Riza suddenly pulled her sidearm, fired several warning shots around him, and showed him where his potty was. He was instantly housebroken, and she named him Black Hayate.
His first appearance was in Fullmetal Alchemist Piece Volume 4 (omake) "Side Story: Dog of the Military", and his first anime appearance was in Fullmetal Alchemist - Episode 13 "Fullmetal VS. Flame". The character has no spoken lines and no specific voice actor listed for the Japanese or US productions.
Black Hayate is the only character from the main story who's first appearance wasn't published in the original serialization, but was in the side story of FMA Volume 4. Black Hayate is named after the Nakajima Ki-84 Hayate fighter plane. In one fan poll, Black Hayate ranked in 23rd place, and tied with the villian Lust.
Black Hayate was being walked by Riza in Central at night while she went to the store, and she was attacked by a living suit of armor called Barry the Chopper. She was in no danger from him and easily subdued the killer.
Riza had Hayate keep Fuery company while she took on her sniper's position during the "Fishing Mission" to lure out the Homuncli. When she was attacked by Gluttony, Hayate leapt on him and bit as his neck to let Riza get free. He jumped off on her command so Riza and Fuery could shoot him. While she and Roy Mustang followed Gluttony, she commanded Hayate stay to protect Fuery.
Edward Elric came to visit Riza after all the commotion with the hunting of Homunculus. Ed accidentally spat out coffee over Hayate's face when Riza teased him about being in love with his childhood friend Winry Rockbell.
Riza brought Hayate along on her unofficial mission as part of Mustang's planned coup. She had trained Hayate to know and signal if they were being followed. He could also identify human from Homunculus. Kain Fuery jokes that he deserved a rank for his work, so, Roy gave him the rank of 2nd Lieutenant, a higher rank than Fuery.
After the Homunculus and Father were defeated, Hayate had met a female white dog that he had several puppies with.
Black Hayate was given strict obedience training by Riza Hawkeye to follow her commands. He stayed when commanded, and was fierce in protecting his master. She had also trained him to recognize if someone was nearby or following her and could give an alert.
Hayate's animal senses could also allow him to sense when one of the Homunculus would appear.