The "First Order Heavy Scout Walker" Lego set depicts this multi-legged vehicle participating in the battle on Crait from The Last Jedi:
But this does not seem to match the events of the movie, and I don't recall seeing this vehicle on Crait at all.
I'd considered that it may be one of the vehicles seen
pulling the battering ram cannon,
but this answer identifies those as the "AT-HH" / "All-Terrain Heavy Hauler".
According to Wookieepedia, the Last Jedi Visual Dictionary designates it as the "AT-HS" / "All-Terrain Heavy Scout", but as I don't have a copy of the book, I can't confirm this.
So, what is this vehicle, actually? Was it cut from the film? Was it designed for the film, but left unused in the production process? Was it merely designed to provide a mid-sized vehicle for toys and Lego sets? And regardless of its origins, has it appeared in any Star Wars fiction outside of The Last Jedi?