This illustration is based on a Narwhal with two tusks found in 1800. It is from an 1806 Biology book “The British miscellany, or, Coloured figures of new, rare, or little known animal subjects.” by James Sowerby, page 17. The following is an excerpt from the book regarding the image.
This animal was cast on the coast at Friestone, in Boston-Deeps, on Feb. 13, 1800. It perfectly agreed with the name given by Linnaeus [description of narwhals], in having but one tooth looking like horn; but, on examining the upper jaw, it was very evident that the other tooth had been lost; and we have since seen a perfect skeleton of the head of this animal with the two teeth fixed in their proper sockets.
James Sowerby
The book refers to the narwhal as a sea unicorn, which is quite adorable. The book and image are in the public domain, Biodiversity Heritage Library is the source.