Aphmau made him a mount dog, which means that she could ride him around.
He was one of five dogs that Aphmau owns, the other four being Thorgi, Puma, Cookie, and Jiggy.
As of season 2, he passed away before Aphmau got back, but before he passed he had dalmatian puppies with an unknown female dog. He now rests in a grave within Scaleswind, along with the other dogs.
Phoenix had a bright blue coloured collar and is most likely to be male. They had the default Minecraft dog skin otherwise; grey and white fur with black eyes.
He has very little personality, and like the rest of Aphmau's dogs, is generally playful and active.