The "blue devil" is an unknown spider, likely in the Lycosidae family, said to inhabit at least one cave near Black Rock, Belize. The oldest, and only known mention of this cryptid, was featured in an article in Animals and Men, a quarterly magazine on cryptozoology.
The article was written by arachnid researcher (and chess master) Carl Portman, who, in 2013, visited the Black Rock lodge in the Cayo District of Belize. Carl's guide, Carlos, told him of a cave high up in the mountains that was inhabited by "huge blue spiders." Despite Carl's heart condition, he, Carlos, and his wife took a hike to the cave. In the cave, they found whip scorpions, frogs, lizards, bees, and even Mayan pottery, but no blue spider.
Then, Carlos went a bit further in the cave and managed to find one of the blue spiders, resting on a rock. Carl then took a few photos before it scurried into a crevice. The spider was a striking and slightly iridescent royal blue colour, with a small, ovular cephalothorax, long legs, and large pedipalps. It was the size of a man's hand, and it had small front-facing eyes and vertical fangs.
The blue devil's anatomy suggests it is some type of wolf spider, albeit an undescribed species. Although a species of animal native to only one cave seems far-fetched, various cases are known to exist. Examples include the Tumbling Creek Cavesnail, native only to one stream in a cave in Missouri, and the Devil's hole pupfish, endemic to a single flooded cavern in Nevada.