When you think of Narnia, you might picture creatures of all shapes and sizes that live within that world. But where exactly do these beings originate? Unlike some authors who design their fictional worlds around just a single mythology (e.g. the Middle-Earth of Tolkien) or their own unique ideas (Aewiar from the Wingfeather Saga anyone?) Narnia is an amalgam of fantastical creatures not contrived from C.S. Lewis’ imagination but, rather, taken from the myths that he read and enjoyed.
Dryads, centaurs, and fauns! Oh my!
Many peoples you’ll encounter while reading the Chronicles belong to the Greco-Roman mythologies. In fact, the first Narnian you meet is a faun, Mr Tumnus. He even plays a flute-like instrument just like Greek god Pan, who is often depicted as a faun. 
Centaurs, a rowdy folk in Greek mythology, became the wise star-gazers who were greatly respected by all. Interestingly, Chiron, (the only true noble Centaur in the Ancient world), embodied many great traits identified with Lewis’ own centaurs and was a great healer like Cloudbirth from The Silver Chair. 
Mythical gods such as Silenus and Bacchus are mentioned by Tumnus in The Lion, the Witch and the Wardrobe, and they, along with the River God, make an appearance in Prince Caspian.
In almost all of the books, there are references to many different types of Nymphs, the semi-divine spirits of the trees and waters that can embody themselves in the form a woman. 
Although inspired by ancient mythology, with many of these beings, Lewis decided to change some of their characteristics, in order to better suit his narrative and also, perhaps, his younger audience.
Narnia and the Norse!
Lewis loved the Northern myths as he grew up and he continued to love them even long afterwards because of the fleeting Joy it had given him – “ Jack threw himself into studying Norse mythology, hoping to experience Joy again.”  It is no wonder that he would bring the giants, dwarfs, and dragons of such legends into his own grand tale.
Tall and fierce and perhaps a bit dull-witted, the giants you may imagine could hardly be classified with the Nordic “giants” called jötnar.  Rather, Lewis derived his giants from the Anglicized derivation of the jötunn. 
The mining “Sons of Earth” have always been associated with good craftsmanship beyond any other being. Despite being the probable origins of Lewis’ dwarfs, those of Norse stories are not quite alike with his. Especially considering the small stature of his own dwarfs compared to the possible human-height of the Nordics’. 
“Full of greed, Fafnir changed into a dragon to guard his treasure…”  Sound a little familiar? Dragons and greed have almost been synonymous, starting with this Norse legend; therein was the idea of Dragon-Eustace and the Lord Octesian’s selfish lusts for the hoard of wealth.
Originally considered a real animal, recorded in histories by the Greeks, the unicorn made its way into medieval stories and legends.  While an indigo horn may have been C.S. Lewis’ preference for the beast, horn colour does not seem to be specified.
The sea-people or merpeople have very murky origins. Originally without a tail in Northern European folk-lore, just as Lewis described the Sea People Lucy sees while aboard the Dawn Treader, it was adopted later as an influence of the Greek mythos’ Siren. 
One of the most interesting creatures are the Monopods/Dufflepuds which were not invented by Lewis at all. Single-legged creatures with one giant foot (called skiapods or monocoli) had been described first by the Greeks and later in Medieval bestiaries.  It can only be assumed that Lewis found one of the entries and decided to use this odd little creature in his own work.
From the Greek and Nordic myths that Lewis enjoyed to the more obscure fantastical creatures this mixture he provided makes Narnia a unique world that will likely stand the test of time.
 Etymonline. Mermaid.