Because they didn't have any intelligence and maintained their original form, Sea Monsters could live for a long time and become extremely powerful while gaining unique characteristics. They would either grow to unbelievable sizes or were highly toxic. Some had magical gifts as well.
In the Grand Sea, Sea Monsters mated with the Demons, evolved and became smarter. They were to build their own homes and create their civilization. Despite of being classified as part of the Demon Race, they conserve their own characteristics and consider themselves a race apart.