If you mean in general there’s a lot of ideas. If you’re allowed to base it off of mythical creatures and pre-existing myths:
-for anything horror-themed, a Wendigo is many people’s go-to. Ever read Stephen King’s “Pet Semetary”— personally my favorite book by him? Wendigos basically have to do with cannibalism and causing (in humans) “insatiable greed and hunger,” so yeah there’s a lot to do with that.
-Taking mythical worlds from ancient times is always cool. For example how Percy Jackson centers around old Greek mythology but, of course, is its own story— the myth basically provides a setting and characters. Celtic Mythology, Norse Mythology (recognize the name Thor? That’s from Norse!) and West African Mythology all, in my opinion, are extremely interesting and provide a realistic basis upon which you could write about these (real!) worlds and simply create your own story within.
-Legends having to do with classic children’s tales. Put a twist on the boogeyman— or make him real and horrific. Or maybe write about monsters beneath beds. And it doesn’t have to be scary, take it from monsters inc — they wrote about a common fear and made it into something else.
-Things to do with personal children’s stories. Monsters under beds are common fears, but less common and more specific fears include that of dolls or other specific story-like things; Chucky is an example. Not all children fear their dolls but in a Chucky story they sure would. Put a twist on any kids’ ideas; dolls, teddy bears, play-pretend, imaginary friends, etc.... these are all legends to children which can be made real through writing.
-The legend of Zelda... sorry I had to say it. But if you could write about it it would make for a cool story.
-Legends about heroes and such. For example, maybe Superman wasn’t as super as he seemed (The Boys has a similar plot). Or perhaps Leprechauns are actually evil! Maybe Santa Claus has menacing intent, maybe his gifts come with a price— or maybe coal isn’t all he gives to naughty children... common legends exist all around us, if you have trouble just look into any holiday or popular children’s tale tbh.
-If you’re allowed to be more free you could write about epic, fantastical places like Atlantis, Olympus, El Dorado, and Asgard! All cool places with cool stories.
-If you’re allowed to make it up / have to make it up, legends often center around some form of truth; so you could start off with a small ‘real’ thing— in the case of Dragons, they could have originated from ancient people’s interpretation of fossils or perhaps ancient, giant lizards, lol. But you could take any normal thing and simply spin a huge exaggeration of it, making dragons from lizards. Even better, if there was once something you misunderstood, you could pretend that misunderstanding was real. (When I was younger I used to think that Clowns were another species, maybe because of their noses? But that makes way for a story about clowns whose red noses are protrusions formed from years of evolution which turned out an entire race of humans known as “clowns,” perhaps in a horrific way...?)