Just a quick idea for a monster, whom i am currently naming the 'sticker', as i have no imagination. Anyway, this mob would hang off the top of large caverns and caves, and shoot down it's acidic goo at you, which would slowly deal constant damage to you and slow you down, or give you some kind of de-buff with similar attributes. It would have low health, and could drop down on you if you were to stand beneath it, and cling onto your head, quickly depleting your health. I have made a quick little pixel art (if you can really call my pictures 'art'). It's drops would be some of the in-game currency (pixels I believe), possibly only 2-3, maybe more if their common to get. it could drop bile, which can be bottled and thrown at enemies, doing similar de-buffs to the goo it fires at you. it could be found in radioactive areas, and vary in size, but it would mainly be large enough to fit on a player's head, and small enough not to look ridiculous while doing so. well, just a little idea, I'd like to see some feedback on it.
Not a bad idea man, though I would probably suggest making it something you can avoid if you are smart, maybe having it make a not-friendly sound before "firing" and having a somewhat slow projectile.