Well I beg to differ. In the Pern books they are actual creatures...they are in fact genetically bred off of indigenous species on a colony world. Almost everything about them is described. McCaffrey wanted to write a Sci-Fi story. The science is in there, it works...I even roughed out a working process for converting P2H4 to PH3 (phosphine) in the "fire stomach."
BUT...that is SF. As far as fantasy goes, my scope is limited, and I have seen novelists run the gamut from feasible to "WTF?!".
I'm a big fan of Carl Sagan's ideas on exobiology, where the idea is to figure out "how does it work," rather than simply "does it work at all."