The Campaign Builder's Guild

So I recently cursed one of my players so they will have to raise a creature from its baby stage on, what would be some of the best and most terrifying creatures to have to raise?

Assuming you're talking about D&D or PF or some d20 game like that, any sort of high-CR evil creature would be pretty terrifying, and it would raise some interesting moral questions as to whether the creature was truly "inherently evil" or not. If not, it could grow up as the party levels up and give them a pretty unusual ally!

Baby aboleth or beholder. The former because of aboleth's Lemarkian memories, and the latter because I'm assuming (and hoping) the PC-caretaker doesn't look anything like the ultra-xenophobic baby. Also, cuddling the latter risks horrible transmogrification, and the latter would have looks that can kill. Literally.

A Purple Worm, Manticore, or Yrthak could be fun.

An Otyugh would be gross and kind of funny - it's always stopping to eat garbage.

I like Rose-of-Vellum's suggestions a lot.

You could always be really mean and make it a Rust Monster. It could be all cute and adorable even, until it eats a player's +2 flaming sword or something.

Actually maybe don't do that.

I think I'm going to use the beholder idea. I'll keep you posted on how things progress.

"Mommy, daddy, you look strange. Wrong. Let me take a closer look at you..."