To be clear what happened here - I usually don't farm old raids for mounts - 11/02/16 was the first day of the second reset after patch 7.1, when the raiding for leashes achievement for ulduar came out, and I was in there for pets. I didn't get a pet the entire run, then the mount dropped.
Under nearly identical circumstances, 2 1/2 years later, I'm not getting any pets to drop and thinking it would be funny to get the mount again... and it happened.