Deep ones are new monsters added in SLASH'EM and dNethack. There is a large level difference between the deep ones and deepest ones, so once you are experienced enough for deep ones to appear you will likely encounter them in some form until the end game. This also means that if a lower level character manages to tame a deep one (which is not difficult as their MR is only 10), they will eventually have a level 45 pet deepest one. All deep ones are capable of treachery, but will eat corpses and thus are relatively easy to keep tame.
The deep one is the only form that is capable of wearing body armor and cloaks, and all forms can wear all other types of armor. If a deep one wearing body armor grows up into a deeper one, the body armor is not destroyed as with polymorphing into a larger size, so it is possible to raise a deepest one that wears a full ascension kit of armor.
Deep ones are capable of swimming and will also be frequently generated on the Plane of Water.
Deep(er, est) ones play a much greater role in dNethack. They are abundantly generated on the Anachrononaut quest, complete with firearms and other future weapons. They are also generated in the Neutral Quest, in R'lyeh and the Lethe. Deep and deeper ones can be spawned in a dNethack throne room, with a deepest one as the overlord.
The deep one and its related kin all come from "The Shadow over Innsmouth" by H.P. Lovecraft, creator of the Cthulhu mythos.
They are intelligent ocean-dwelling creatures, and are fishy/froggy humanoids in appearance. The transition from deep one to deepest one in-game is a reference to the deep one-like giants in Lovecraft's earlier short story "Dagon." Deep ones are immortal, and in the wider Cthulhu mythos it is accepted that they grow continuously throughout their lives. The oldest deep ones, therefore, would be very large indeed, thus explaining both the human-sized deep ones in "Innsmouth" and the whale-sized giants living in the deep ocean in "Dagon."
For eighty thousand years Pth'thya-l'yi had lived in Y'ha-nthlei,
and thither she had gone back after Obed Marsh was dead.
Y'ha-nthlei was not destroyed when the upper-earth men shot death<
into the sea. It was hurt, but not destroyed. The Deep Ones
could never be destroyed, even though the palaeogean magic of the
forgotten Old Ones might sometimes check them. For the present
they would rest; but some day, if they remembered, they would
rise again for the tribute Great Cthulhu craved.