Below are old school stats for a number of cryptids, aliens, beings, and spirits from real world cryptozoology, ufology, and conspiracy theories for if you just really want the flatwoods monster in your game.
A note on statblocks: while this is compatible with whatever osr system you wish to use, they are formatted a bit differently from some systems, and there are some stats that assume houserules I use.
Hit Dice: # of d8s rolled for HP. Armor: if present, points subtracted from damage.
Attacks: number of attacks per round, attack bonus, damage of attacks
Saving throws: Toxic save/Paralysis saves/Magic save/Dodge save/Device save
(note: toxic is what is usually called poison, dodge is breath or dragon breath)
(second note: its roll equal to or under for success)
(third note: use dodge save instead of armor class)
Move: movement rate in feet per turn. Morale: number out of 12 for morale checks
Hit Dice: 3 Armor: 0
Attacks: 1, +3, d6 damage
Saving Throws: 14/13/15/12/13
Move: 120' Morale: 7
These frogs are humanoid amphibians that hide in sewers and under fetid bridges, congregating in small groups of 2 to 5. Loveland frogs, named after the first town they appeared in, are unspeaking and have unknown motives, crawling and hopping through their stinking homes during the day and only venturing out at night. They all are found carrying sparkling wands that illuminate the area around them with the loud sparkles, and they are able to paralyze creatures with these wands by pointing them at the victim, freezing them in place on a failed paralysis save.
Hit Dice: 4+1 Armor: 2
Attacks: no attacks
Saving Throws: 16/15/16/13/15
Move: 100' Morale: 10
Star mummies are beings from another world with unusual appearance. They are vaguely humanoid, with grey wrinkled skin and legs fused together into one limb, their gangly arms ending with crab-like claws. Their heads are featureless except for thin slit mouths and their noses and ears nothing but pointed peaks. They float 2 feet above the ground, and travel about in great silvery craft. Star mummies steal away their victims into their spacecraft, but they do not incapacitate them to do so. Rather, if they "stare" at any sort of being, and said being fails a magic save, then they are almost "charmed" by the being and will accompany them into their starship.
Hit Dice: 6 Armor: 2
Attacks: 1, -1, 1d4 damage
Saving Throws: 17/16/16/14/14
Move: 90' Morale: 12
An almost spectre-like being from another world, its bulb-like head and metallic skirt immediate identifiers. While it can attack with its sharp claws, its main mode of offence is the stinking pestilential cloud that it can produce from under its body, and the disease which it can inflict. It takes a round of combat to release this cloud, and those within it take 1 point of damage per round from the sting to their eyes and to their lungs. In addition, those who enter the cloud must make a toxic save or contract the "Flatwoods Fever". Those who contract it are confined to their bed, oozing black goo from their orifices for several days if not weeks.
Hit Dice: 3+2 Armor: 0
Attacks: 2, +4, d6 damage
Saving Throws: 14/15/8/14/8
Move: 130' (flying), 110' (walking) Morale: 6
Batsquatches are large apes native to cold northern rainforests and to mountains near them. They are bulky and muscular, and in addition to being large humanoid apes, they have leathery bat wings. Both the hands and feet of a batsquatch are opposable, often grabbing at things with their feet while they fly through the air.