The lost Original Drawings - Plenisner
Steller wrote: "the living beast, strange as it is true, ... [in] its form, movement, and habits..."
Regrettably he has not left us a decent image of this beast by his own hand, as he "was utterly incapable of making recognizable drawings"*)Stejneger 1936: Appendix A. Steller therefore was used to be accompanied by a qualified scientific draftsman.
As there was no space on the St. Peter for additional scientific staff, Vitus Bering agreed that, during the sea voyage to America, Corporal Plenisner should be Steller's draftsman if needed. Friedrich Plenisner was crewmember on Bering's vessel St. Peter as Kyrillic=painter = painter and surveyor*)Büchner, S. 7, (Steller called him Conductor). During the sea voyage Plenisner and Steller became friends.
On the 12th July 1743, after the safe return of the survivors, Steller sent his manuscript "de Bestiis marinis" on the annual supplyship from Bolscheretsk*) on Kamchatka's westcoast to St. Petersburg, together with a number of drawings which most certainly*)Büchner, Stejneger Plenisner had drawn after nature on Bering-Island. Among others were two illustrations of a female seacow, i. e.:
Table  I  representing the animal prone on its belly.
Table II representing the animal, lying on its back, so as to show the form of the lips, mammae, genitals and tail.
Table III the Masticatory Plates and the 'Insect on that Animal' (insecta hujus animalis).
Due to the insecure postal system on the long way to Europe, each relay station routinely made copies of important documents, before these were sent on. One copy of the manuscript reached St. Petersburg, the drawings however did not arrive with it and must be considered irretrievably lost*)Stejneger 1936: Appendix A.

Friedrich H. Plenisner
The German-Baltic (Courland) Plenisner (? - 1778) was:
1730/35 Cavalryman
transferred to Yakutsk.
1737 was banished for life to Ochots for an unknown delict,
(there Corporal of Ochotsk Harbour?)
1738 already Vitus Bering recruited him for the Great Nordic Expedition.
1741-41 took part in Bering's voyage to America, as painter/surveyor (?),
During the voyage drew coastlines of Ochotsk, Bering and Alaskan sea (?)
threafter ban was lifted (?).
From 1745 Soldier (unknown rank and branch) in Moscow.
1753, back in Eastern Siberia, in the Garrison of Anadyrskogo (as Governor of a prison?).
1760 in the rank of Lieutenant Colonel Governor of the Anadir Region.
Since 1761 Commander of a fortress (?).
1762 Colonel, Commander of Okhotsk and Anadyr.
1763 and 1764 sent expeditions to the East Siberian Sea (Bear Isölands)
collected geographic and ethnologic data of the Kuriles, Chuktschi and Kamchatka for the Academy of Sciences St. Petersburg.*)Source up to here: www.kamchatsky-krai.ru and A. Erman, 1855, Archiv für wissenschaftliche Kunde von Russland, Page 217.
From 1776 until his death 1778 he lived (retired?) in St. Petersburg. probably met Peter Simon Pallas there, on which occasion he could have handed him a drawing of the seacow, the "Pallas Picture" (see second next page)*)Source last para: Stejneger 1936, the Pictures of the Seacow.
Evidently Plenisner had no formal drawing education.

(This biography is incomplete, further details and corrections are welcome)