Not sure what the full-fledged Mythosaur will look like but if we're assuming it could feel the vibrations of him breaking the surface and has something like a tentacle, it could have wrenched him down to have a look and then tossed him aside when he wasn't something to eat. Din was weakened by whatever the hell that cyborg was doing to him and blacked out from the trauma. If this Mythosaur was the same one ridden by Mandalorians in time past, maybe it recognized Bo's jet pack and didn't try to eat her.
Feels like there's probably going to be a Deux Ex Mythosaur at the end of the season when the Empire shows up, and either Bo or Din (pulling for Bo) winds up fulfilling the prophecy and riding it while holding the dark saber.
Life is better with a beagle