Shalosh tannoth are a possible inhabitant of a strange-larva-hellish seal. They are not particularly threatening one-on-one, though they can become somewhat threatening if allowed to build up an army of offspring.
Flesh hook attack
The tannah's primary attack traps the target in a flesh hook, which restricts movement much like a bear trap. Flesh hooks can immobilize flying targets as well as grounded ones.
Each turn, a shalosh may (1/9th chance) produce 1-3 strange larvae in adjacent squares. These larvae will quickly grow into either vrocks, hezrou, nalfeshnees, or mariliths, complete with appropriate (organic) equipment. Because these larvae do not grow into more shalosh, there is little danger of the situation snowballing out of control; however, it is still wise to quickly kill the shalosh.
Tannin (plural tanninim) is an ancient Hebrew word meaning "sea monster" (or sometimes "serpent," and sometimes translated as "dragon"). Ancient Semitic religions used sea monsters to symbolize primordial chaos, and sometimes evil (see also Tiamat/the Chromatic Dragon.
The etymology of tannah is a bit less clear, however, it seems to be the feminine form of tan ("jackal"). It appears to symbolize desolation, loneliness, and abandoned places. English-language sources tend to claim that it means "dragon" and is the feminine form of tannin, but this appears to be a mistranslation. Regardless, dNetHack uses the word as though it were the feminine form of tannin.
Shalosh is a Hebrew word meaning "three."