Q:

Why is Species 8472 called Species 8472?

This is my current understanding of the situation.
The Borg were the first to encounter Species 8472. Presumably, they called them Species 8472 because they have encountered 8471 species before them. Then, everybody else calls them Species 8472, because that's what the Borg called them initially.

Is what I've said so far correct?
What does Species 8472 call themselves?

A:

Yes
Nothing in canon, but they are called the Undine in Star Trek Online.

