The Mega Mutant Serum is from the Bored Ape Chemistry Club, also a part of the Ape Family. The collection includes 10,000 Mutant Serum NFTs airdropped to all Bored Ape NFT owners. The holders can then use the Serum to mutate their Ape. Several Mutant Apes are already sold in the market. All Serums expire upon use. 

According to a Twitter user, there are currently just 3 Mega Mutant or M3 serums that have not been used. This is because Yuga Labs had originally airdropped only 8 of them. The other more common serums include M1 and M2 types.