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Tethys (also spelled Tethis) is a "sea troll" made entirely from ambulatory water. Trolls native to the Discworld are mineral products, but this alien troll seems to be an elemental (in the sense of "the 4 elements", not a periodic table element) instead. Through him, one can discern tidal patterns, and several different species of fish.

Tethys makes his living scavenging off what rocks up against the Circumfence - the barrier that encircles more than 3000 miles of the edge of the Disc, and which nominally belongs to the Arch-Astronomer of Krull. He briefly captured Rincewind and Twoflower.

Tethys is an alien to the Discworld, and the only "sea troll" on the Disc. He used to be a "meatman" (the water-world equivalent of a fisherman-- he hunted wild game on praries) on his home world of Bathys, where he fell off the edge, froze solid, and then thawed out as he approached the Disc's Sun. He has considered jumping over The Edge to find his home world again, but has never found the courage.

On Roundworld, Tethys is a Titanness from Greek mythology.

One can't help wondering whether TP would ever have written Tethys into existence by about book 5 - an elemental troll is not really in keeping, and has never been mentioned since...

Of course, Tethys is the only sea troll on the Disc and it's quite possible that "sea troll" doesn't mean "a troll from the sea" anymore than a prarie dog is a dog or a sea monkey is a monkey or a sea horse is a horse.

In The Dark Side of the Sun, Tethys is also the great ocean on the world of Widdershins and has been colonised by the intelligent water-being Chatogaster.

It is entirely possible that the Tethys/Tethis of The Colour of Magic is a later refinement of the intelligent water-based being Chatogaster, who is capable of assisted travel between planets, and represents a recycling of ideas between The Dark Side of the Sun (1976) and the Discworld series.