...our common room lies behind a concealed entrance down in the dungeons. As you’ll see, its windows look out into the depths of the Hogwarts lake. We often see the giant squid swooshing by – and sometimes more interesting creatures.
--Gemma Farley, Prefect from Slytherin House (Pm)
The giant squid is a gigantic, relatively friendly cephalopod which lives in the Hogwarts Lake.
The largest invertebrate known to Muggle science, the giant squid can grow to 70 feet in length. While the giant squid is a deep salt water creature, at least one giant squid lives in the freshwater lake near Hogwarts castle, and seems to be more or less friendly toward the inhabitants.
The giant squid at Hogwarts:
Giant squids are not magical creatures. Curious, isn't it, that Rowling chose to put this particular non-magical creature at Hogwarts, of all places. On the other hand, this particular giant squid certainly doesn't act like a normal one, so it would certainly seem to be at least somewhat magical.