So, Meppa's name came up in PON (I think it was TTT), in reference to a specific cant known to the Cishaurim. It was called "the Meppa Cataract" and was mentioned (only once) as being used by the Cish as one of their dispensations of water. I don't think that was a coincidence. Following on from that, he had to have been established as someone important/powerful in the Tribe of Water. Assuming Meppa is, indeed, the one for whom the Cataract was named, his identity was unknown to the Schools outside of Fanim lands. He was never mentioned by the Scarlet Spires as one of the Primaries of the Cish. I suspect he is another son of Moenghus, though as to what his role is within the confines of the Thousandfold Thought is obviously still a mystery. Moe likes those names that begin with 'M', though... Moenghus, Mallahet, Maithanet... Meppa?