My Dog (1840)
attributed to
Sir Edwin Landseer
Although Sir Edwin Landseer owned many dogs during his life, I have found no evidence that Landseer ever owned a Newfoundland.
In her article "A Tale of Black Beauties," Emma H. Mellencamp discusses this work (which is in fact an engraving, not a painting) and notes that it was one of a series of three engravings which an unscrupulous printer had constructed by lifting images from extant Landseer paintings. (Unscrupulous and not particularly interested in factuality: one of the three pirated engravings is titled "My Wife" — despite the fact Edwin Landseer never married! The woman in that engraving is the niece of a noblewoman who was an occasional patron of Landseer.) The engraving entitled "My Dog" is simply the head of the Newfoundland portrayed in A Distinguished Member of the Humane Society with some vegetation added by the engraver.
The full text of Mellencamp's essay, originially published in Newf Tide (vol. 9, no 1; 1978), is available at the Newfoundland Club of America website's history section. Many thanks to MaryLou Zimmerman for bringing this article to my attention.