Marh was a Trill symbiont who lived in the Caves of Mak'ala on Trill in the 14th century. It had a rare biochemistry that would have rejected any Trill host, which made it unsuitable for joining.
A cautionary tale about Marh and the Trill Guardian Aylim was told to new Guardians in the Caves of Mak'ala as late as the latter half of the 24th century. The story goes that the diligent and affectionate Aylim was sad for Marh as Marh's biochemistry didn't allow for it to be joined, so it would miss out on the experiences that all the other symbionts would have. Aylim convinced himself that Marh's inability to be joined was a mistake and attempted to join himself. Unfortunately, the bio-chemical imbalances in Marh caused it to reject Aylim as its host and Aylim died as a result. Although Marh didn't suffer physically, it was mentally scarred by the brief joined experience and was never happy again.
The tale of Marh and Aylim was told to Rantic Lan by the Guardian Timor. (ST - New Worlds, New Civilizations short story: "Aversion")