The recent episode S6 E4 “On Your Marks” of My Little Pony: Friendship is Magic has a short scene of a painting lesson in a studio. There are eight ponies in that scene. We (the fans of the show that is) know who seven of those ponies are, and I'm asking here who the eighth pony is.
First, let me list who the other seven ponies are. The first two are easy. One pony is the main character of this episode, the only one important for its story. The green mare with red mane in the center, who serves as the model, is Tree Hugger, the character who was featured in S5 E7.
Three of the ponies are expies for famous real world painters. Although their appearance also resembles that of the real world artist, we can identify them more certainly because each of them is shown with a painting that parodies the style of that real world artist.
- Specifically, the yellow stallion with short white mane and three colored cubes as a cutie mark is Pablo Picasso, and he is shown with a portrait in Picasso's style.
- The gray earth pony stallion with black mane, a long thin black moustache curling upwards, a similar tail, and a deformed pocket watch for his cutie mark is Salvador Dalí. He is shown with a painting of three deformed copies of Tree Hugger, one hanging from a tree branch. The painting and his cutie mark both resemble Dalí's real world painting “The Persistence of Memory”.
- The pale red earth pony stallion with glasses, white mane, and an open can of apples is Andy Warhol. His painting shows four identical shaped copies of Tree Hugger in different vivid colors, and is a reference to a real world painting depicting Marilyn Monroe. The cutie mark and the pony's hairstyle are references too.
Two other ponies are one-off characters who had spoken lines in S4 E12 during the Party Planner song, but have not or barely appeared since.
- Specifically, the light gray earth pony stallion with dark blue hair, paintbrush cutie mark, and a snout that looks like that of Mr. Cake is selling paint to Pinkie Pie in S4 E12. She's wearing a hat in both episodes, but different hats. He was voiced by Brian Drummond and credited as “Art Supply Vendor”. He has the fan nickname Pinto Picasso (really unfortunate now that we have a real Picasso).
- The light green unicorn mare with two-colored yellow hair and red striped shirt is on screen right after the paint vendor in S4 E12, selling a banner to Pinkie. She is voiced by Tabitha St. Germain and credited as “Banner Vendor”. Her fan name is Peachy Pitt.
That leaves us with just one pony in the background in the above image. It is a green unicorn with big white beard and moustache, thick white eyebrows, white tail, yellow eyes, blue dweomer, wearing a pale bright yellow shirt and a black painter's hat. We only see it from far away in the background, partly covered, and never see its cutie mark or painting.
Since the other seven ponies refer to something, it seems likely that this eight one isn't an incidental background pony either. (Mind you, that's what I said in my last impossible to answer question as well, so you shouldn't trust me.) Who is he?
Please don't just give a name as the answer, give some evidence on what matches this green unicorn.