That is an odd kingbird. The all white outer web on the outer tail feathers is found only in Western, but the bill looks too large, the yellow too rich, and the white grayish throat too restricted for Western. I'm thinking possibly a Western X Tropical/Couch's hybrid. Kingbirds in Tyrannus are known to hybridize to some extent.
It just seems like a bird that's the furthest you can get from a non-Western yellow kingbird. I'd have thought a hybrid would at least look more intermediate between the two, and not as strongly towards Couch's/tropical.