I think it depends on what story you need to tell. Creatures are amazing, whether you create one yourself or if you do something with a known monster.

The great thing about using a known quantity (dragons, elves, dryads) is that there are fans built-in. Readers have a connection to these magical creatures that they've encountered before. I can't tell you how many times I've picked up a book in a bookstore or somewhere else because it mentioned dragons. It doesn't have to be a typical dragon with scales and requiring hoards of treasure. New twists are incredibly fun to read. Dresden Files is another place where you can see magical creatures twisted into all kinds of things- each vampire or fairy you run into isn't what you thought from your childhood stories and a potential for disaster. Forbidden Sea was a YA with mermaids that I read semi-recently.

What's the best part of creating your own creatures is that your reader doesn't have any idea what they're in for with your plot. You can do anything with your story and the reader better hang on while you take them for a ride. They have no expectations about your creatures other than what you give them in the narrative. But... then... I could list a bunch of books in that other paragraph, but I'm not sure I can think of anyone who has created all the creatures from scratch. [Help, please?!] Valdemar series by Mercedes Lackey comes to mind, but only because she used real animals and imbued them with greater intelligence as well as creating a few hybrid creatures based on other things - deer = dyheli? They had a few other differences.

Using some of these together is another possibility. Again, Dresden Files.

It could be easier for your first novel to use what speaks most to you. Don't force yourself to create (or not) new creatures as your inner vision of this story goes. It isn't always to write what you know (because technically I have never met a dragon) but it is to write what's in your heart. The rest can be researched and imagined.

Good luck!!