Start with a list of mythological creatures or characters. Unicorns, ogres, mermaids, little green men, even the mighty capybara (ok, fine they only seem mythological). Try to go beyond the first three or four thoughts and dig a bit.
Find some way to ground them, something unexpected they have to deal with in their mundane, every day lives. Ogres have to cook, unicorns have a hard time finding a decent hat, maybe the capybara is annoyed it actually exists when it should be the stuff of legends?
What holds these creatures or characters back? What keeps them from fulfilling their wildest dreams? What are those dreams?
Take a look at Matthea Harvey's The Straightforward Mermaid on the New Yorker, then start writing.