Monster: Eurypterid, Feersum

The recent discovery of a new species of 'sea scorpion,' or eurypterid, pushes evolution of the the Paleozoic predator back approximately 10 million years.  The new genus was named Pentecopterus, after the penteconter, an early Greek galley. Although these particular fossil individuals were approximately six feet in length, specimens of over eight feet long have been found.

The eurypterid was not a scorpion, but an arthropod more closely related to modern spiders, lobsters, and ticks.  The 'sea scorpion' nomenclature come from the spiked tail in some genus', however they have not been found to be venomous, and likely preyed on trilobites and other creatures using modified forelegs and claws.

Source

The animals prowled in shallow, brackish water, and freshwater lakes, via crawling and swimming.

So, what happens if your PCs are, for some reason, wading in a primeval lagoon?

Eurypterid, Feersum:

No. Encountered: 1d8+1
HD: 1, 2 for large specimens (10%)
AC: 7/12
Atk: Claws x2 (1HD - 1d4: 2HD - 1d4+2), Successful hit grapples victim. If wading or swimming, save or be tripped/dragged under.
Save: 17-16
Move: 9 (swim)
Opportunistic hunters/scavengers, they will converge on a single victim. Each additional successful grapple causes successive saves at -2 per eurypterid.  Successful hits will dislodge the beasties.  Any victim submerged by the eurypterid(s) also must contend with drowning damage.

With enough drawn butter and garlic, though, they are quite palatable.

Them's good eatin'