Sea serpents are a kind of sea monster either wholly or partly serpentine. Sightings have been reported for hundreds of years, and recent work by Bruce Champagne indicates that there have been 1,200 or more all told. Sea serpents have been seen from both ship and shore, and by multiple people at once, groups that sometimes count scientists among their number. Despite the numerous sightings, though, no credible physical evidence has been recorded and it is currently believed that the serpents do not actually exist.
In Greek mythology the Sirens (Greek Σειρῆνας) were Naiads (sea nymphs) who lived on an island called Sirenum scopuli, or in some different traditions,some place them on cape Pelorum others in the island of Anthemusa, and others again in the Sirenusian islands near Paestum, or in Capreae which was surrounded by cliffs and rocks. Approaching sailors were drawn to them by their enchanting singing, causing them to sail into the cliffs and drown.
In Babylonian mythology, Tiamat is the sea, personified as a goddess, and a monstrous embodiment of primordial chaos.The depiction of Tiamat as a multi-headed dragon was popularized in the 1970s as a fixture of the Dungeons & Dragons roleplaying game thanks to earlier sources associating Tiamat with later mythological characters such as Lotan and others.
An Elf is a mythical creature of Germanic mythology/paganism which still survives in northern European folklore. In Norse mythology they were originally a race of minor gods of nature and fertility. Elves are often pictured as youthful-seeming men and women of great beauty living in forests and other natural places, underground, or in wells and springs.
The griffin, griffon, or gryphon is a legendary creature with the body of a lion and the head and wings of an eagle. As the lion was traditionally considered the king of the beasts and the eagle was the king of the birds, the griffin was thought to be an especially powerful and majestic creature. Griffins are normally known for guarding treasure and well valued priceless possession.
Unicorns are magnificent and noble creatures that have enchanted young and old across the globe and through the ages. They are both a symbol of purity and goodness and the personification of untamed freedom. Many cultures throughout the world have their own version of the unicorn myth, but most depict them as white horses with a long horn extending from the forehead. The horn is usually spiraled, which makes light dance across the body of the animal as the sun shines down upon it.
For the motherlode, try this comprehensive list on Wikipedia to get ideas. Another great tip - get inspired by folklore from your culture, for example Nordic, Native American, Chinese, Aztec, or Slavic stories!
I'll be posting some more inspiration pictures as the contest progresses.