Tentacular

Giant and Colossal Squids are soooo cool. That creatures as huge as these have existed for hundreds, probably thousands, of years without our knowing much of anything about them makes them even more interesting.

They've been the subject of myth and legend for centuries giving rise to the story of Kraken.

Kraken are legendary sea monsters of gargantuan size, said to have dwelt off the
coasts of Norway and Iceland. The sheer size and fearsome appearance attributed to the beasts have made them common ocean-dwelling monsters in various fictional works. The legend may actually have originated from sightings of real giant squid that are variously estimated to grow to 13–15 metres (40–50 feet) in length, including the tentacles. These creatures normally live at great depths, but have been sighted at the surface and reportedly have "attacked" ships.

We know from the beaks recovered from sperm whales' stomachs that they exist. And in 2007 the largest known one was captured off the coast of New Zealand. They aren't the biggest animals in the ocean, but they're the biggest mysterious ones.