Well, plants and animals if you prefer! Alien flora and fauna is often a thing in DW fic, and even sometimes Earth flora and fauna. Peri-fic is a good one for plants, because author's often use her being a botany student. Plus, in art you get a lot of stuff where characters are drawn with or as creatures/plants (like those things a while back where all the Doctors were owls).

And the prompts aren't tropes as such, they're just a mix of prompts to be freely interpreted.