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.