Diamond Dogs are a species of canine-like creatures that inhabit the underground tunnels of Equestria. They are subterranean creatures, and seemingly spend most of their lives underground, looking for various gemstones to devour.
History
Season 1
Three Diamond Dogs appear as antagonists in "I'll Be Dog-Gone!", in which they steal Pearl, Amethyst, and Lapis Lazuli, and store the three in their personal gem horde. Once Tanzanite appeared though, the three were quickly taken out.
Season 4
The same diamond Dogs reappear, and are revealed to have somehow made it to Homeworld, and had been collecting numerous Gems there for some time. Celestia found the three, and quickly stopped them and brought them to the Diamond Authority.