What struck me about "The Beast in the Cave," H.P. Lovecraft's first completed and second published story, was just how feeble the title monster is! It is almost as if H.P.L. was writing an epic game supplement geared initially for starting level characters and progressively working his way up to the world-wrecking monstrosities like Cthulhu that are his hallmark. I have, in any event, made the following monster as tough as I could, operating under the assumption that the protagonist who slays it has Str 15, gets a critical hit with a thrown rock, and scores maximum damage, knocking it down to -1 hit points and causing it to bleed out. This creature could, of course, be made tougher with the addition of character levels, better ability scores, etc. There did not seem to be a lot of other elements from this story that could be adapted into game terms, unless someone wanted to do a mini-adventure that expands upon the cave colony H.P.L. alludes to in the story and which includes several such creatures. (Illustration ©2012 by Sergio Monfort) 

Cave Beast: Medium Humanoid; CR 1; HD 2d8; hp 9; Init +2; Spd 30 ft. (6 squares); AC 12 (+2 Dex), touch 12, flat-footed 10; Base Atk +1, Grp +3; Atk +3 melee (1d3+2, claw); Full Atk +3 melee (1d3+2, claw); SQ Blindsight; AL N; SV Fort +0, Ref +5, Will +1; Str 14, Dex 15, Con 10, Int 12, Wis 13, Cha 8.

            Skills: Climb +5, Listen +6, Spot +6, Survival +6. A Cave Beast receives a +4 racial bonus on Climb skill checks.

            Feats: Alertness.

A cave beast has completely white skin and appears "to be an anthropoid ape of large proportions ... Its hair [is] snow-white (and) surprisingly thin, being indeed largely absent save on the head, where it [is] of such length and abundance that it [falls] over the shoulders in considerable profusion." The limbs of such a creature are bent in a way that allows it to travel on two limbs or four as it sees fit, to crawl through areas that might otherwise be difficult to get into, and to climb well. "From the tips of the fingers or toes, long rat-like claws extend ... The hands or feet [are] not prehensile." They do not have tails. Its eyes are "deep jetty black" and "deeply sunken in their orbits [and] entirely destitute of iris. ... The nose is quite distinct" and the lips thick.

A Cave Beast can speak a rudimentary language that sounds like "a kind of deep-tone chattering."