Between ancient myth and modern pop-culture and urban folkore are the sort of fanciful mythical figures, often associated with children's stories and holidays. They're a mishmash of pagan beliefs, Victorian mores, and greeting card mascots.Various media like The Santa Clause, The Guardians of Childhood and various Rankin Bass specials have established something of a pantheon, and these codified versions would be most familiar to Americans.
There's Santa Claus, maybe the most well-known legendary being . Consumerism had diminished in Medieval America, but stockings will still occasionally be stuffed. He's a little more mercenary, and the gift-giving transactual, so that the better treats you leave out, the more impressive gifts you get.
The Easter Bunny is the springtime counterpart to Santa, and although chocolate is now a luxury, eggs are relatively easy to come by, and actually, one of the more common snack foods.
The Tooth Fairy is the third member of the gift-giving triumvirate. Of course, children aren't going to get legal tender just for losing a tooth. In fact, for many peasant families, they usually don't even have money per se, and live on a barter system. They are told to plant a tooth for good luck.
With Father Time, and Mother Nature, you have two personifications (And old man with a scythe and hourglass, or a woman decked in foliage) that show up in a great deal of Neo-Medieval art and stories. That is, a character may plead with them for favors, and run afoul and get their comeuppance. Jack Frost, a relatively recent personification of winter, is also fairly well known. It's quite possible a specific embodiment of autumn may appear in Medieval America, as the seasons in the Northeast in particular are very distinct. The Sandman survive as a good excuse as to why we wake up with crust in our eyes. There's also Death, or the Grim Reaper.
The two beings from classical mythology that remain the most iconic outside of the context of Greek or Roman settings are Cupid and Neptune. Their associations with the Pagan past are, if not forgotten, somewhat dissociated with the general layman. Cupid is seen a cherub who is responsible for the tender passions, and maybe even an excuse for sexual improprieties. Neptune is a somewhat revered/feared figure for sailors, and the father of all mermaids.
There's also the legend of the stork, as well as the groundhog. Everyone knows there's a groundhog, and works like the movie Groundhog Day has probably given the creature a mystic power, though it's unlikely the name Punxsutawney Phil has really stuck.
And of course, there's Uncle Sam, the personification of America, which is now divided, but in the non-Denominational Church he's exists as a saint and rugged warrior. Both Saint Patrick and Saint George, Merlin and King Arthur. He's the patron for the 4th and July, and a nostalgic, wistful icon of what was.