Dude, this is seriously the “rat, giant (Sumatran)” from the Monster Manual.
Here’s some trivia – the (Sumatran) part of the entry header comes from a throwaway mention in one of Sir Arthur Conan Doyle’s “Sherlock Holmes” stories. In “The Adventure of the Sussex Vampire”, reference is made to “…the giant rat of Sumatra, a story for which the world is not yet prepared.” I’m certain Gygax was aware of the line, and that made its way into the header.
It's also referenced in a Firesign Theater bit, which may be more directly what Gygax had in mind.
That guy must be at least a first level fighter, to handle that half hit die monster without failing a morale check.
Found in the cellar of an Inn perhaps? worth a few gold to eradicate for a simple barman? First adventure here I come.
I'm pretty sure that is a Nutria
https://en.wikipedia.org/wiki/Coypu (Coypu, Nutria, River Rat): 1.5-2 ft body plus 1-1.5 ft tail