More than 10 years ago, I was on a flight SFO to ORD, pmUA, on a 757-200, sitting in the second exit row (last row of E+). There was a group of 6-8 retired people traveling together, sitting about 2-3 rows behind me. One had a little dog in a carrier. It wouldn't fit under any of their seats. The FAs were trying to help them but ended up getting a GA to come on and explain that the choice was between either not taking the trip or taking the trip on a later flight that day (I think on an A320 that had wider seats). The one person with the dog left the plane to get rebooked. The others in the group stayed onboard. A standby took the seat of the dog owner. I overheard the FA tell them that this happened on the 757 regularly. The aircraft in this tragic situation is the 737-900, which has similarly narrow seats.
JK-SFO