Giant aquatic scorpions. From the CBC:
The researchers said the scorpion lived between 460 and 255 million years ago and would have been among the top predators in its environment, feeding on early vertebrates and smaller arthropods...
...Some geologists believe that the giant arthropod evolved due to high oxygen levels, while others argue that they evolved in an "arms race" alongside their prey, the early armoured fish.
Arms race? How did that work? "Oh, so you've got armour. Yeah? Well I'm freakin' huge". It's at this point in the fantasy where all the other fish start chanting "Served... served... served".
I'm pleased that among its many other marvels, evolution also managed to make the world less terrifying. The 2cm spider outside my window makes me vaguely uncomfortable. An 8ft sea scorpion would make me insane.
If you can get past the 46cm claws, our giant friend here may actually be an efficient and carbon-neutral form of transportation.