What monsters did you dms take from folklore and made them terrifying as hell for your players?

Bicorn -- they are absolutely terrifying to Clerics since they eat divine stuff.

The metal eating Alicanto. A flock can eat a dragon's hoard of coins.

Hodag: “It had the head of a frog, the grinning face of a giant elephant, thick short legs set off by huge claws, the back of a dinosaur, and a long tail with spears at the end", according to one survivor of an encounter with a hodag. Vicious, sneaky, and predatory, Hodags are a scourge that often sneaks in through patrol lines into civilized territory, usually by burrowing.

Isnashi. Not going to say more, lol. Google them.

Impundulu -- in this case, it is a form of homunculus, and used to feed on others. Unfortunately, hurting one upsets the Hag that usually created it...

Abada -- here, because everyone had a hard time thinking of more traditional unicorns (that were violent, lol) and wanted them to be the Lisa Frank cutesy things, I used the ancient Abada as the replacement -- two horned, more akin to a zebra without stripes than a horse and does not get along with those who would steal it's food source (and they are carnivores).

I use a lot of folklore from different places.  I am able to handwave much of it as Feywild or Spirit of the World stuff. Sometimes I use the story (few Russian, at least a dozen different indigenous, a lot of assorted Xhosa, Yoruba, and similar African, and even some modern stuff like the Jackalope and Vorpal bunny (really just an almiraj).

That's cool, how about some monsters that would fit the theme of "folklore" in y'all's games.

One of my favorite things to do is to research folklore and mythology and then make them into Dnd monsters. The following list of monsters only have brief descriptions, but I strongly advise people to look them up. I have used:

Krampus- Christmas goat demon

Aun Pana- Hairy fish monsters with arms

grootslang- half elephant, half snake

cactus cat- a spiky cat creature that is loves alcohol 

hidebehind- used as an explanation for why lumberjacks mysteriously disappeared. 

agropelter- ape-like creature with whip like arms that pelts woodsmen with tree branches

yara-ma-yha-who- a australian dreamtime monster that sucks your blood through its fingers.

Dokkaebi-korean goblin that likes wrestling

cat-sith- dangerous cat faeries that can steal your soul

qalupalik- an arctic water monster

codrille- disease bringing dragons that go through different growth stages

ushi-oni- evil ox spider spirits

kasa-obake- evil umbrella yo-kai

bake-kulira- ghost whales that seek revenge against whalers.

owlman- cryptid that can control how animals behave

akhekh- dragons that are similar to hippogriffs but throw dragons into the mix

leshy- protective spirits of the wild

ahuizotl- water monsters that have a hand on the end of their tail

psoglav- russian ghouls, they have the legs of a horse, the body of a hairy man, and the head of a cyclops wolf

bauk- serbian monsters that dwell in darkness

polevik- field spirits with grass for hair

fetch- supernatural dopplegangers that appear before someones death

minokawa- giant dragon-like bird

karnabo- grey skinned elephant trunked, basilisk eyed man with paralyzing breath

amixsak- the empty, angry skin of a dead walrus

dulhath- horrible monster with a big appetite that often appears as a man with sharp claws riding an ostrich, it can cause all who listen to its voice to fall down before it and bow.

sazae-oni- mollusk yokai

For more really weird monsters, research yokai, fearsome critters, or pick something like slavic mythology.