Actually, if memory serves, there are many giant squid in the lake, and they're hunted by a tribe of mermaids(check the fourth book). I can't wait until the fourth movie, let's see them not mention the squid now!
Rowling's usual accompaniment on the kid's best sellers list is Lemony Snicket, with his Series of unfortunate events. It's up to eleven books, and the eleventh book features some unfriendly octo-like creatures. The cover shows some tentacles in the background, some with suckers, and others that are less friendly.