What Sara♥Jean said. Come back to it. If you've spent 15 minutes, you get credit. Sometimes you have to walk away and either wait for that 2am revelation, or wait for that revelation that will come when you're working on some other loosely related (or wholly unrelated) assignment. I was working on my climax and drifted off on a tangent. When I realized I was drifting, I inserted some spaces, wrote ***RESEARCH LATER***, then got back to my climax. Maybe you can do something similar since you can't research at the moment. Just stick to the brainstorming, organize your ideas the best you can, and make notes reminding yourself to research harpies / sirens / centaurs later.

I'd also like to add that since harpies, sirens and centaurs are (arguably) fictional, they can have any traits you decide to give them. Let's say you decide that a siren's call is heard in your head, not your ears, but later you learn that it's commonly accepted folklore that the call is actually heard in your ears. Who cares??? In your story, the call is heard in your head. You don't have to change it.

Cheers,
Michelle