Fred is 23 (I think) but looks older than his age. His hindquarters are weak - he has a bit of wasting - which may be from when he had Lyme disease when he lived out east. When he canters, he doesn't go very fast, and does a bit of a "bunny hop" with both hind legs almost moving together. He may have just become exhausted from the running, and didn't have the muscle strength to rise when he rolled. Fred's owner said she was glad she wasn't there to see it - she hates to see him getting old.
We've recently observed some odd behavior in Misty. She's always been one of those horses that tends to become wobbly when girthed, and if you're not careful, she actually becomes unconscious and falls down. Her owner keeps her walking when girthing and that solves the problem. Recently, Misty has been falling deeply asleep in her stall after evening feeding, head drooping to the ground - and then she just falls over backwards, with her hindquarters hitting the stall wall - sometimes she sits down. This wakes her up, and she gets up. We're not sure - she may be narcoleptic - but it isn't due to an inability to lie down and get up (and therefore get enough REM sleep) that causes narcolepsy in some horses - she can easily lie down and get up. She's an Impressive-bred Quarter Horse and a HYPP carrier (not expressed), but I doubt that has anything to do with it.