I personally feel the answer to this would be fairies, but that is me. Let me know what you think are some of the most neglected in literature.

Probably creatures like Griffins, Cyclops, Minotaur, or many of the mythical Greek creatures.

Greek mythological creatures, unicorns (how many YA books are there on unicorns???)...
I have to agree with fairies a little bit. At least I haven't read to many modern day YA books with them. But I know those books are out there...still, there could be more.

I think fairies aren't over-looked. I've read a ton of newer books about fairies lately.

I think the ancient Egyptian creatures deserve to be not so over-looked.

I know of at least one YA book about a unicorn, and I know of many others in which they are mentioned.

I would say Golems, especially in their old Yiddish form. Also, Al Ghul would be an interesting change, although ghouls do turn up in The Graveyard Book by Neil Gaiman.

I think the Namba Pamba Pah Poozi is the most overlooked creature of all, because nobody has ever seen it, smelled it, touched or even heard of it until now.

And someday, the hoogula (a mythical creature I made up, who is in my book, The Wish Thief), will be so well known it'll be as generic as a werewolf or vampire. Hey, I can dream, can't I?

Absolutely you are allowed to dream. And as for my opinion on the fairy thing, I would just like to see something a little more grown up about fairies....the same with the unicorns too. They've been so much apart of children's lit that I think they deserve a little more attention.

“Absolutely you are allowed to dream. And as for my opinion on the fairy thing, I would just like to see something a little more grown up about fairies....the same with the unicorns too. They've been so much apart of children's lit that I think they deserve a little more attention.”

Read 'Lords and Ladies' by Terry Pratchett.

Mythical Greek/Roman/Egyptian creatures. Definitely not monsters of more recent legend. (Vampires, werewolves, etc)

Leprechauns??

At least, I haven't read anything except for old Irish children's stories.

but as for mythical/fantasy creatures of the past fifty-odd years... I have to say...

the Barbie doll

(Seriously. She's a freak creature, yet there's not one story about her growing an extra head, or being an alien killing machine. Yes there's lots of princess Barbie and fairy Barbie stories, but not one story that is as freakish as she is. Grossly overlooked heh heh!)