Before the Shoot (1849)
by
George William Horlor
detail from
Before the Shoot
Horlor (1823 - 1895) was a British painter known primarily for sporting and animal subjects.
Can I be 100% certain that the dog depicted in this work is a Landseer? No — especially since the horse in this painting is a pony, making the dog look larger than it otherwise would — but it's a reasonable enough possibility (given the range of appearance of "Newfoundland" dogs in the mid-19th Century) to merit inclusion here.