Octopuses, which for the record is the correct pluralization of octopus. (It can also be octopodes, since the word is Greek in origin, but never octopi.)

Blue Bloods

“For starters, octopuses have literal blue blood. There’s a common misunderstanding that human blood is blue inside your body when it’s deoxygenated, but that comes from the fact that your veins look blue through your skin. Deoxygenated blood is still very red because of the iron-based mechanism by which our bodies transport oxygen molecules. Octopuses said ‘no, thanks’ to iron blood, though, and swapped in a copper-based protein that binds oxygen instead. It’s more efficient than iron in the cold, low-oxygen environments that most octopuses live in. It sure does make them spookier, but they’re not alone. The ocellated icefish has clear blood and there are lizards that run green. Both are from Earth.”

Octopuses’ brains are in their arms

“Two-thirds of an octopus’ neurons reside in the long appendages (tentacles). This decentralized way of thinking means that even severed arms can “think” for themselves, or at least respond to physical stimuli and try to escape whatever is trying to eat them, which is why people die from trying to swallow live octopus arms only to find that the arm is still fighting back (a reported six people die this way on average each year in South Korea, where the dish is popular).”

Intelligent 

But their peculiar approach to brains hasn’t stopped them from ranking among the most intelligent creatures that we know of. Octopuses regularly use tools, solve puzzles, and generally cause mayhem by sneaking in and out of their enclosures. They also sometimes accessorize by hopping inside old coconut shells and using them as little mobile homes, all while looking more stylish than most humans.

Suckers and 3 Hearts

As they travel, they also taste everything that they walk on since their suckers are all sensory organs. You’d think that would motivate them to swim everywhere, but unfortunately one of their three hearts has to stop beating whenever they swim, which is quite tiring and means that many octopuses prefer to stroll. Their other two hearts provide blood to the gills, but that third heart circulates blood to the central organs.”

“The main organs reside inside the octopus’ bulbous head (called a mantle), which contains no bones. The only truly hard part of an octopus is the beak, which is basically its mouth. This means that the critters can squeeze through almost any opening as long as it’s bigger than the schnoz. Everything else is negotiable.”

Short Life

“But perhaps the weirdest thing about octopuses is that, unlike many of the other highly intelligent creatures populating our planet, they don’t live long. Some live just six months, others a few years, and most males die shortly after mating. The females last long enough to protect their clutch of eggs, during which time they slowly starve to death.”

https://www.popsci.com/octopus-aliens#page-2