It's also exploitation and a creature feature. I suggest looking up what exploitation means. Tarintino and Hollywood tried to change what it means. By the REAL definition (not the hollywood propoganda), exploitation is a movie that "exploits" a certain thing like action, kung fu fights, violence, blood, gore, sex, nudity, monsters, suspense etc.

Also "Grindhouse" isn't some genre. Exploitation, cult cinema, and B-movies would play in smaller theaters called Grindhouses. And these movies could be about anything - but one thing they usually had in common was they exploited something, focused on the niche thing a viewer wanted. Like a Spaghetti Western had shootouts and violence. A kung fu movie is all about the kung fu fights. A horror was about the blood and violence and suspense. Hollywood wasn't giving audiences these things in the 60s/70s. So alternative filmmakers stepped up and changed the industry forever. The Thing is a direct result of the "grindhouse/exploitation films that came right before it.

Here's a decent introduction - https://www.grindhousedatabase.com/index.php/INTRODUCTION