Hey, is there anyway to mass tag something?

For example, adding a species tag to a specific character?

I believe there are mass tagging tools out there, but I would advise against it as you can easily mistag posts.
Go through each individual post and add in the appropriate tags. Remember to tag what you SEE, instead of what you KNOW.

You'll have to manually add it to posts because of images with alternate_species.

Your best bet is going to be a search for "charactername -species", then just add it to anything that doesn't have it that should have it.