I think I remember it was something with a special quest, but I'm not sure....

It's indeed via completing a quest from Merana that you can turn a pet Hydrus. You can find her on the Omen Islands, and you need to have completed an Uncharted Treasure Map to get access to them.

: sigh And again this clenches teeth Treasure Map D: I guess its really expensive.... Well, thanks anyway :)

Actually, during the pirate/island plot the uncharted pieces were given out massively, so right now they're still very cheap. I'd suggest now would be a good time to buy them. ;)

: Okay, thanks, I'll try to get them asap :D

EDIT says: Hehe, my Plasmabluete is nice, isn't she? :3