The mutants were sorcerers who once inhabited Whateley Manor and withdrew from Innsmouth.
Background[]
The folk of Innsmouth were powerful sorcerers. They were former humans, but due to their dark magic rituals and worship of old ones (such as Cthulhu) they had transformed into fish-like creatures. At some point they withdrew along with Whateley Manor into the World of Daventry.
Most ran into the sea and disappeared immediately upon arriving in Tamir perhaps heading for R'lyea. They spoke little; they left no tales. The ones who stayed on land gurgled what little information that had been passed down, and then locked themselves inside the mansion. They disappeared from sight, and soon the eerie glows and rays which stabbed from the house at night vanished also. In time the hauntings began.
Wilbur Whateley was also believed to have been a mutant, but not of the fish-like variety.
At least one fish mutant ended up in The Swamp of Tamir where it feeds on the frogs and other creatures of the bogs.
Behind the scenes[]
This is a reference to the fish people is a reference to the Deep Ones in Shadow over Innsmouth.
There actually is a fishman of the Creature from the Black Lagoon living in the The Swamp of Tamir in KQ4. Accessible if one turns into a frog while in the swamp.