Cut-52371-A is the code given to a currently unnamed Human that was within the White Diamond Space Station as one of White Diamond's experiments.
Appearance
On first appearing in Season 4, he appeared much like a normal Human being, with a stitched scar on his chest, with his identity number engraved on his shoulder.
By Season 7, most likely due to its experimentation, he now has draconic limbs along his body, including very small wings, a reptile-like tail, and moving on four legs instead of just two. Part of his skin was cut away, coincidentally the same patch of skin that had his identity number.
Personality
He's not as self-aware as normal humans, behaving more like a aggressive animal than a person. This behavior made him animalistic by his appearance in Fillydelphia.
History
Season 4
While some of the Crystal Gems were trying to escape from White Diamond, they stumble upon the experimental chambers, where they briefly see Cut-52371-A in his containment cell. He tries attacking them, ramming the glass, but was unable to break free.
Season 7
In Fillydelphia, he was sent down there (presumably), and started causing numerous fires within the city, to which the police force suspect the local Dragon population. He wasn't discovered until during a stakeout by By The Book, and combined with the Crystal Gems and Mane Six's help they manage to drive him away.
Abilities
Fire Breath
Presumably due to his dragon-like anatomy, he is capable of breathing fire.