Where on Disc did Tethis go?

Question

In The Colour of Magic, Rincewind and Twoflower meet Tethis the Sea Troll, a being from Bathys, another planet, who fell to the Disc. Tethis and Twoflower get into the Potent Voyager, and Rincewind ends up hanging onto the outside, as the Voyager launches.

In The Light Fantastic, which follows on directly, Twoflower and Rincewind fall back onto the Disc (sort of; arrangements were made: if memory serves the Great A'Tuin does a barrel roll under the influence of the Octavo and sort of catches them), but there is no mention of Tethis, not in this novel nor, that I remember, in any other. Ever. Nor did Terry Pratchett ever mention him in interviews / online from what I can find.

However, I haven't read every Discworld publication (the final novel, the YA stuff, nor most of the companion books such as the diaries or maps), and my searching skills aren't stellar.

Is there any canon information on what happened to Tethis? Or, in the absence of that, any really good speculation (I am, of course, not looking for discussion; but there's a lot of magic on the Disc, and from established events there may be a decent explanation even if it was never outright said in canon).

NOTE: I am open to the possibility that Pratchett simply thought "bugger it" and pretended he didn't exist. These are the first two instalments after all, and the author admitted he was still getting to grips with the setting (check out the Patrician in the first books, compared to the last). But hopefully, if this is the case, he mentioned it somewhere.

Answer 1

Unknown.

As you say, Tethys was never mentioned again in any later Discworld book nor mentioned by Pratchett in interviews. The Terry Pratchett Wikia has this to offer (emphasis mine):

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 prairies) on his home world of Bathys, where he fell off the edge, froze solid, fell through space for some time and finally thawed out as he approached the Disc's Sun, and landed widdershins of Krull, where he was eventually captured and made a slave. He has considered jumping over the Edge to find his home world again, but has never found the courage.

Tethys' fate is unknown. It is presumed that he entered in the Krullian spaceship after Twoflower, but when Rincewind and Twoflower were brought back to the Disc by the Change spell, the sea troll wasn't there. Maybe he somehow found his way back to the world of Bathys and could live in peace ever after with his wives and children.

It's also possible that Pratchett later decided he didn't really want to have such a character and that this explains the lack of further mention of him (a 'passive retcon', if you will):

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 with Discworld physics and chemistry, and he has never been mentioned since ...

Answer 2

Near Skund (probably)

Although Tethis' ultimate fate isn't mentioned in the novels (he never receives another mention after The Colour of Magic), he does get a passing nod in the Ultimate Discworld Companion. It's presumed that he ended up roughly in the same place as the Potent Voyager.

Tethis was a rather squat (although his height altered with the tides) but not entirely ugly old troll composed of water and little else. He was a pleasant translucent blue colour, with cold, fishy breath and a voice that made people think of submarine chasms and things lurking in coral reefs.

He briefly left the Disc with TWOFLOWER and RINCEWIND and has not been heard of since. The POTENT VOYAGER, the ship on which they reluctantly travelled, eventually landed in a lake near Skund. Presumably he is still there.