Several published adventures that I intend to use in my Frost-Bitten Lands campaign (http://www.autarch.co/forums/actual-play/my-acks-campaign-setting-frost-bitten-lands), such as Skull Mountain and Sinister Secret of Saltmarch, have Lizardmen as a major ingredient. However, I’m not sure if these lizardmen - the natural inhabitants of warm swamps - would fit my frozen campaign.
So which creature should replace them? Kobolds (which are warm-blooded)? Human cultists? Warm-blooded lizardmen (ala theropods)?
Sealmen, ottermen, arctic fishmen, lizardmen with feathers (like dinosaurs) or lizard men that live by hot springs, can’t venture far from them and have bloody bloody wars over the heat sources.
Also: elephant seals fight as ogres. A lair of elephant seals have females and young equal to 100,000% the number of males. The females fight as orcs and 20% of the young die being crushed by the male who just doesn’t give a fuck.
There are virtually endless swamps in the far north, and you have several good traditional choices to replace Lizardmen. Mermen are one possibility, as northern riverlands are one of thier more common locals, but a better choice might be Koalinth, since they are amphibious, prefer the cold waters of the north and are much like lizardmen in character. You could also go with Frog Men (or both at different times). Both Koalinth and Frogmen debut in Supplement II, BTW.
Just a note on amphibian species, but the most obvious amphibian that actually needs and enjoys cold climates are terrestrial salamanders such as the Fire Salamander and Alpine Newt, etc. Most frogs, toads and lizards are going to want warm and humid climates, but these salamanders have very slow metabolisms and survive harsh winters by burrowing and can go for weeks without food.