It's said that scientists who work with squids and octopuses fall in love with them: not in an
anime sense, but ascribing all sorts of
dolphin-like beauties to them. I can understand this. I too have been fascinated by the idea of land cephalopods.
Giant land cephalopods, of course.
Intelligent giant land cephalopods, of course.
Squids have got wonderful things like jet propulsion, invisibility, giant tentacles, and huge razor-sharp beaks. I believe that some of them can launch themselves above the water surface and fly, using a jet burst. This is scary enough. Now combine them with all the best genes from octopuses.
The blue-ringed octopus has one of the deadliest poisons of any creature. An octopus in a laboratory tank can squeeze itself into a tiny space and escape, oozing through unbelievably small cavities. And they are reported as using tools. This hasn't been confirmed by repeated observation, it's true, but they have been seen picking up a stone and putting it inside a clam shell to prevent it shutting, so that they can make a meal at leisure. Octopuses pass intelligence tests as easily as rats and pigeons.
How did our own fishy ancestors make the transition to land? Lungfish that could survive with a bladder full of air when the pools dried up, and that could waddle through the mud from one pool to another -- a bit of a Just So Story, but it'll do for our purpose. Now a squid is probably too streamlined and oceanic to get anywhere near the littoral, but I can easily imagine octopuses lying in wait in shallow rock pools. Heavens, the blue-ringed octopus does.
Give it a more leathery skin, so it doesn't dry up on exposure. It crawls around the seabed on its cute little legs already, so that's not a problem. Let it start by slithering over the seaweedy rock-flats pulling up limpets: that's not such a jump in fitness space.
To make it permanently ambulant on land we'd have to overcome the skeletal support problem. Well we could postulate some new development -- but let's say a simple reversal of the homeobox genes could give it the endoskeleton of a cuttlefish. But broken up. Articulated, joined by cartilage. Or exapt the radula: make it longer and longer, curl it round, break it into pieces, until it can act as an endoskeleton.
Then you've got giant invisible flying poisonous super-intelligent multi-tentacled carnivores. R'lyeh! R'lyeh!
Two little things to add since originally writing this. Cephalopods aren't as continuously active as mammals because their blood contains the copper-based haemocyanin instead of our iron-based haemoglobin, and this stores oxygen more efficiently than it can release it. Secondly: the name says it all, Vampyroteuthis infernalis, the Vampire Squid from Hell.