It's a mod called monster girl enemies. Changes a bunch of enemies into monster girls (slimes, all spiderlike enemies, wolf, shark, unicorn, etc) edit: From the mod page, here's what it changes:

Still rough around the edges, but definitely going to be a front runner for awesome mod content going … I played terraria years ago but quit when there were sexual references, and i wish i could play without any of that. Is there a mod for this game that removes anything that isn't family-friendly? If not, then is …