Avoid using this tag!

"Cross species" is a vague term — here are less ambiguous tags covering all potential interpretations:

alternate species

For characters being depicted as a different species than they normally are (compare "crossgender")

hybrid

For a character that is a "cross between" two or more species

interspecies

For characters of different species engaging in sexual activities with one another

species transformation

For a character turning into another species ("crossing into" that other species)