Uh, yeah... he was, shall we say, a 'wide load': a very low-to-the-ground terrier mix - 'portly' maybe is the term of art.

This was in Y. Under-seat space is very different depending on aircraft, and I actually found the dreaded RJs might have the most room - of course, I also got away with being in the exit row with him on the RJs which is definitely not allowed, but the CoExpess FAs maybe are less clued in. The 757-200 did not have enough room at all. Those are the only two that I remember details on.