Aliasing monster_boy → monster_girl_(genre)
Link to alias

Reason:

Monster boys are the gender-swapped version of monster girls, they are monsters too and popular in anime.

What about monster boys that are not genderswaps? This alias would lead to monster boy searches being flooded with monster girls instead.

To be honest, I think both tags should be removed since they're animal humanoids for the most part and it's so badly tagged that it's difficult to tell what exactly falls under it. I've even seen images of Splash Woman under this tag before.

EDIT: I looked into it further and it seems those tags are actually redundant. "female (species)" and "monster_girl (species)" produce similar results. Ditto for "girly (species)" and "monster_boy (species)". See for yourself.

demon female vs. demon monster_girl
demon girly vs. demon monster_boy

And for evidence of inconsistent tagging:

etna monster_girl vs. etna -monster_girl

_{^{Side rant: Who the fuck is this "docu" and why does he have so many character tags?}}