Buckbeak is a hippogriff who was sentenced to death for hurting Draco during Hagrid’s first Care of Magical Creatures class. Harry and Hermione figure out a way to save both him and Sirius, which also provides Sirius a way to stay on the run from the Ministry. He eventually comes back to Hogwarts’ herd, under the name Witherwings.
Buckbeak is composed of “buck” (a horse’s form of defense) and “beak” (part of a bird’s anatomy, which can be used to attack), relating to both halves of a hippogriff‘s body (specifically dangerous parts), as well as making a nice alliteration.
His alias Witherwings, also alliterative, refers to his ability to fly, with maybe a bit of weariness.
Bonus: Rowling’s scrapbook contained a list of names for hippogriffs: Stormswift; Hothoof; Fleetwing. (source) Those could either be alternative names for Buckbeak during development, or names for other hippogriffs in the herd. The three of these relate to either flying, danger, or horses, and two of them are alliterative.
