Diamond's Dog is the name given to an Alien creature sent down by White Diamond. It only has one appearance throughout the series.
Physical Appearance
Diamond's Dog is described as a strange quadrupedal creature with frail limbs, though as big as a rhino and as long as a bus. Its body resembles a large dog, with a tail very thin like that of a whip. The head has armor plating like a shield, but lacked any external ears, nose, or eyes, but still has a set of large jaws. Its teeth are like razors more than actual canine teeth.
History
Season 6
It only appears once in the episode "The Results Are In", sent down by White Diamond in order to apprehend Steven Universe. The creature seemingly was under some form of control, and attempted to kill them all, only it didn't attack Steven until after he gave himself up to it. Upon given the word, it attempted to flee with him, but was caught by Connie Maheswaran. It dropped Steven, and quickly turned on Connie, its brutal attack breaking her arm to such a degree that it was planned to be surgically removed.
It was not seen since, and it's unclear what exactly happened to it, though according to the writing it was indeed defeated by the other Gems shortly after this in some way.