Another thought about these threads:
I was thinking that in most cases except for the fantasy staples (like orcs, unicorns, dragons, etc. . ) any of the strange or "invented" monsters are going to especially fit a role in how the DM sees the setting and the tenor of the campaign in general.
While a giant fire-breathing lizard with wings does not seem weird to us b/c we are so used to the idea of dragons, a shape-shifting fungus that looks like a battlemech might stretch our suspension of disbelief, because we haven't grown up with it.