Twilight Sparkle was performing a time travel spell (perhaps after Starlight Glimmer at the end of Season 5) and she and her dragon assistant Spike winded up in the Eocene period (55 to 35 million years ago) where they encounter the first documented Equines, the Eohippus and a small group it seems. The painting in the background was done by renowned American artist Charles R. Knight, who did many awesome pictures of dinosaurs and other prehistoric life for museums and old magazines such as National Geographic in the early 20th century. While much of his art is outdated thanks to modern paleontology, he did inspire countless other prehistoric artists and even inspired the dinosaurs in movies like The Lost World (1925), King Kong (1933) and the Rite of Spring sequence in Walt Disney's Fantasia (1940). His art is amazing and his late grand-daughter Rhoda even created a website for his art if you want to check it out- http://www.charlesrknight.com/