Clint's team! I can't say I'm all that familiar with Clint's history other than what I've read in fic, but here's what I got as far as his Pokemon go.
He got Pidgeot as a Pidgy very early on his life as they were fairly common around the circus, always eating leftover crumbs left by the attendees. The one he found was injured when he came across it, and he nursed it back to health.
Noctowl was found as a Hoothoot during an espionage mission through a forest. His Pidgeot (a Pidgeotto at the time) was the one who befriended it first, somehow managing to convince the Hoothoot to help out Clint. After that it just stuck around.
In my head, Pidgeot and Noctowl have learned how to fly in sync so that Clint can somehow travel back and forth between the two of them midair and shoot arrows at the same time! It is all very, very cool....
And yeah, his Pidgeot and Noctowl are larger than normal, though not by much. Pidgeots are supposed to be 4'11" and Noctowls are 5'1" according to the Pokedex!!
Anyways, Black Widow is next!