That IS a pretty good-sized dip!
Not that you would have really wanted him to, but I’ll bet he could’ve gotten himself out of there if something REALLY scared him. He would’ve scrambled and rolled around SOME way and got his feet under him, even if he scraped himself up in the process. I’ve seen horses get out of places that you didn’t think they could ever get out of when something or somebody put the fear of God in them.
I'd hope so... Otherwise, he'd be helpless as an overturned turtle layin' there.
He needed me yellin' at him and just a little push. Hubby said it's like us, when we get sunk down in a comfy chair. ;~)