Isherwood seems to be busy, so I'll man the post for today.
There has been plenty of stories describing romance between the (usually) man and woman of two different species: humans and na'vi, human and any sort of AI, human and asari/turian/quarian... Rarely has their friendship been explored: how friends notice things in each other that strangers won't, how their species affects their relationships (if at all) and how prejudices may run between them.
Today, write about friends of different species. Use whatever two species that you can imagine, as long as you portray the things that make their friendship special (because of their personality, their biology or other people's perception of either of the friends).