damnlassotool has 28 posts
damn_lasso_tool has 7 posts

Which name should we keep?

The Tumblr title says "Damn Lasso Tool," so my vote is for damn_lasso_tool.

Whatever we choose, the other tag should be aliased, obviously.

If the name has spaces in it when not written as a username, then the version with the _s should be used. So I have to back what Maxpizzle said.

I'd support damn_lasso_tool per Maxpizzle, but ultimately it doesn't really matter so long as they're both aliased.