Name: Oklahoma Octopus
Size: Reports give it the size of a horse, which would easily give it an arm-span of 25-30 feet. Similar in size to some reports of the Giant Pacific Octopus which lives in Puget Sound in Washington State.
Appearance: People describe a creature with multiple long tentacles and reddish-brown in color occasionally rising to the surface, then disappearing under the water.
Threat: Medium to high. It has been reported to grab swimmers and drag them down and the lakes it is claimed to be in has continually reported increasing rates of unexplained drownings, upon which this creature is blamed.
It rather goes without saying that this creature is found in Oklahoma. Specifically, reports of attacks and sightings focus on Lakes Thunderbird, Tenkiller, and Oolagah. Which presents our first problem for the existence of this creature. All of these lakes are man-made and none are older than a hundred years. So creatures that have been there for centuries these critters aren't.
Still, there are legends of the native americans about creatures which lived in the ponds and rivers before the lakes were there, and it is certainly possible that suddenly being given a much larger area to live in with a more than abundant food supply would allow creatures which were previously minor threats and annoyances to become real monsters. That is very possible given the time frame we are currently working with.
The other problem is the water. To date, there are no known species of freshwater octopus and many experts will make a definite statement that it isn't possible. Personally, I disagree with that. Many saltwater species have made the transition to freshwater and just as many animals regularly go back and forth through estuaries and river systems. There are freshwater dolphins and jellyfish, and Bull Sharks have been seen and even caught living miles upstream in pure freshwater. So to say any animal couldn't make the transition is unnecessarily closing off avenues of the imagination. Just because we haven't found one, doesn't mean one doesn't exist.
More than likely, though, this is one that probably doesn't. What is known to be in the lake are introduced giant catfish which match the coloration of the Oklahoma Octopus. Their whiskers could very easily, in a panic, be taken for tentacles and large enough catfish are known to attack and pull down people into the depths to drown. As an introduced species with no natural predators and being fed on a constant basis by fishermen, they are also known to be able to grow to massive sizes and to be completely unafraid of humans.
As expected of a creature this recent, literature and even movies about it are extremely rare. The best I can find is an episode of the Animal Planet "found footage" series Lost Tapes. There isn't even a cheesy SyFy Original movie. Yet.
Still, you never know sometimes.