Fire King is a Tier 12 Pet obtainable through a Tier 12 Egg. It can also be bought with robux for R 0 in the Premium Shop. When bought from the shop, it will have 125 speed, but 700 coins.
When grinding for a Rainbow Mortuus, you will get many of these and the Ice Queens.
Its gold stats are 3 times those of the base pet with 12600 Coin Collection and 300 Agility.
Its rainbow version starts off with 5 times the normal stats with 21000 Coin Collection and 500 Agility.