Once in a very great while, a member of the Sahagin race is born without any scales at all, and much smaller gills than are normal for a member of their race. In centuries past, these abominations were killed as a matter of course. In more recent times they have been repurposed to act as the Sahagin eyes, ears, and hands in the mortal world.
Sahagin infiltrators are rare enough that they don't constitute their own caste, and fall under the rulership of the Sahagin Mystic caste. Paradoxically, they are also the type of Sahagin most likely to be encountered on land.