I think the last Kalpa was watery, and the Dreugh and Sload are left-overs. This is just a guess.
"The Mundex Terrene was once ruled over solely by the tyrant dreugh-kings, each to their own dominion, and borderwars fought between their slave oceans. They were akin to the time-totems of old, yet evil, and full of mockery and profane powers. No one that lived did so outside of the sufferance of the dreughs... Deny not that these days shall come again, my novitiates! For as Mehrunes threw down Lyg and cracked his face, declaring each of the nineteen and nine and nine oceans Free, so shall he crack the serpent crown of the Cyrodiils and make federation!" - Commentaries, Book 4
"From the Provisional House he looked into the middle world to find the fifth monster, called The Ruddy Man. When the dreughs ruled the world, the Daedroth Prince Molag Bal had been their chief. He took a different shape then, spiny and armored and made for the sea." - Sermon 28
Hard to interpret these any other way. If not a separate Kalpa, definitely a Dawn Era offshoot when the world was still forming. I suspect Akavir (though BGS cannot seem to make up their mind about it) can be explained to have similar origins: Still essentially Ehlnofey derivatives, just separated from Man and Mer by time and distant ancestry, existing on one of the offshoot worlds of the latest Kalpa (note that they also worship Tosh Raka, Aka’s mirror brother). In this interpretation, the Dreugh/Sload would simply be some of our most distant relatives. Then again, we reach, more or less, the same conclusion if we assume they are previous-Kalpa holdovers.