Sorrow is a Halloween limited pet, no longer obtainable as of Update 10 on 11/9/2018. It has a rarity of 4 stars, and has 10,000 levels.
Sorrow can be bought in the premium shop for 249 robux. Premium Sorrow's stats are 10,000. The premium Sorrow has more agility than the hatched Sorrow, but the hatched one has more coin collection than the Premium version of it.
When golden, the Sorrow's stats are multiplied to 30,000.
When rainbow, the Sorrow's stats are multiplied to 60,000.