The big headed guy eats sheep and excretes sweaters. As for the long legged dragon dude, he's a species prototype for the Aerie. A strain of wyrms that evolved to be runners rather than slithery swimmers.
Depends on what I'm trying to accomplish and whether or not I have a clear idea. Reference is always good whether or not you're creating an animal combo creature. I could be creating something completely abstract but still find a quick anatomy ref useful.
For the most part, if I want an animal combo I would of course find reference of the animals I'm thinking of. But for these guys and for a lot of my drawings I just doodle, create thumbnails, and refine. The guy on the top of the page is a good example of that, he started out as scribbled abstract shapes. I looked for a creature within those shapes and then refined it into big mouth up there. The guy on the bottom however I had a plan for, I did small thumbnail drawings to try and get my idea down on paper, then took one of those that I liked and refined it.
Other times, if I do know exactly what I want to draw, I do just draw it in one go. But a creature design can always be made better with reiterations.