Some fleeing mobs low on health will begin to warp, resetting their position and becoming very hard to hit. You can see this in the first 10 seconds of the video; the snake moves forward, warps back, moves forward, etc. It's so annoying trying to finish these things off, it's all get closer get closer...
This particular instance took place in South Ro at coords -1531 -670 -76 but I see this all the time in many outdoor zones in various spots.
https://i.imgur.com/lEG7AZy.mp4