Much like the boss version, this Thwomp follows Mario horizontally when on the ceiling, and falls whenever it gets close to him. However, this version doesn't spawn Bob-Ombs, is completely invulnerable, and is perfectly compatible with levels of more than one screen's length.
Some optimisations, SA-1 compatibility and shared subroutines. Nothing special.
Though if you want to use this sprite, I recommend on using less flat terrain, specifically make the path close to the thwomp, or add some waitng points as you can otherwise simply run away from them, making these sprites essentially useless in a bad constellation.
Also, leave them in an empty space and don't block their path as they then float away and move to the top of the screen.