^argh. Those evil ones in Nethack… 
Love the idea.
F- Fuath

Origins: Gaelic Mythology
The fuath is an evil water spirit. It is described as having webbed toes, shaggy yellow fur, mane down its back and a tail whit spikes. It also wears a green dress, robe or kirtle.
Sometimes it marries humans and have children whit some of their properties (Tail, mane and/or webbed toes).
References: Monstropedia, Wikipedia, Medieval feel
I wanted to do a different approach this time and try to go for a medieval-ish style, just for fun.
Thanks for the comments guys 
G-Gandaberunda

Origins: Hindu Mythology
A strong two headed bird capable of carrying an elephant in each of its talons and each of its beaks.
Reference: Wikipedia, Mythical Creatures and Beasts Wiki
H-Hitotsume-kozō

Origins: Japanese Folklore
It has one single gigant eye in the center of its face, and a long tongue. They are rather harmless.
Referance: Wikipedia
I-Indrik

Origins: Russian Folklore
This gigantic beast is the lord of the animals, residing on The Saint/Holy Mountain. It is described as having the body of a bull and the head of a horse whit a horn on its snout.
Referanses: Wikipedia, Monstropedia, Monkey on a bull, Generic bull 1, Generic bull 2 (Huge bull, pic and video) ,Horse (Kabardin-Russian warmblood from mountainous areas) in a book of horses.
J-Jenglot

Origins: Indonesian and Malaysian folklore
Jenglot is a small living, mummified, humanoid doll whit staring eyes, prominent canines and long nails. Its hair keeps growing and some have joined legs. People who own these, feed them whit blood by placing human or animal blood next to the doll, it will “eat” it in its own way.
They are used in black magic to cast spells or give their owner supernatural abilities. But if they are not feed and treated properly, they will turn back on their owners.
Referanses: Monstropedia, Google image search
K-Kaaiman

Half-fish, half human creature that drowns people. It resides in deep pools. It has white skin, long black hair and red eyes. It makes a sound like a crying woman/person.
Referances: Monsropedia, Various sites on google, telling the same story (summed in one)
I wanted to make her stand out and I wanted to make her “beautiful”. From afar, she would look quite human and anyone would save her being too late once they realized she’s not human. If she looked like a beastly fish only the compassionate would save her. The story people tell of it gave me more of an impression that they’d want to save her, and then when they saw the red eyes, they’d rather turn back but couldn’t 
L-Llamhigyn Y Dwr

Origins: Welsh Folklore
It’s described as a gigantic frog, whit bat-wings for front legs and no hind legs. It also has long tail whit a stinger at the end.
Origins: Monstropedia
This was fun to paint, even though I made him in such a dull pose 

I felt like going simplistic today, but maybe this was too simplistic
Hey there, great idea and challenge you have made for yourself! So far you’ve been rocking i like reading the descriptions along with the images. Looking foward to more.
N-Nekomata

Origin: Japaneese Folklore
The Nekomoto is a cat whit a split tail. It can make ghostly fireballs, walk on its hind legs, talk and reanimate corpses. It will haunt any household it is kept in and will eat anything, large or small.
Reference: Wikipedia
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Edit: Thanks Rabid 

Wanted to do this differently, so I drew it in a comic book style 

Not too happy whit this, tried to use a brush I haven’t used in a while and I think I see why 
Q - Qiqirn

Qiqirn is a bald dog spirit whit hair on its feet, mouth, ears and tail. It’s frightening and also skiddish.
One way to scare it is to call out its name.
Referance: Wikipedia
R - Rompo

Found in African and Indian stories
It lives only on human corpses. It has the head of a jackrabbit, whit the ears of a human, front legs of a badger and hind legs of a bear, its body is like a skeleton.
Referance: Wikipedia

Trying to recapture the two that got behind. Friday I got caught up in something else. Yesterday I had a massive headache. Might leave T for tomorrow, but I have a little hope that I’ll cook up something today 
T - Tikbalang

This half-man, half-horse misleads adentureres. To find he way back they have to walk whit their shirts inside-out. Its knees rise above its head when it squats
Ref: Encyclopedia Mythical, wikipedia
U - Unwaba

Origins: Zulu Mythology
A mythical chameleon sent by the sky-god to tell humanity they’re immortal. It used so long time that humanity was no longer immortal.
Referance: Encyclopedia Mythica, Google image search
Such a cute and goofy creature. Easy pick for the letter u 
V - Vanara

Origins: Himdu epic Ramayana
Its a forest dweller quite similar to monkeys. Their bodies are covered in bright feathers that are usually brown.
Referance: Monstropedia
W - Will O’The Wisp

Who hasent heard about this creature?
The articles write more about the phenomenon surrounding it: Wikipedia, Monstropedia
X - Xing Tian

Origins: Chinese mythology
Xing Tian is a geadless gigant whit eyes on his chest and a mouth on his navel.
Referance: Wikipedia. Chinese shield–
Not at all happy whit it. I feel I could do so much more whit him, but I don’t feel like fidgeting more on this monster.