Not all posts have to be entirely serious…
Just as the woolly mammoth and woolly rhinoceros are larger, hardier and more powerful than their warm-weather cousins, so also is the woolly wyvern larger, stronger and hardier than the ordinary wyvern. Its body and wingspan are 30-40% greater than that of the ordinary wyvern (my ordinary wyverns have an 18-20ft wingspan and a 6-8ft long body, but your wyverns might be different) and its entire body, save for its muzzle, is covered in a thick coat of ruddy-brown hair, similar to that which covers a woolly mammoth. Only in its flame is it inferior to its hot blooded cousin: the ice and snows do not nurture a hot fire in the same way as the baking heat of the desert. The fire is still hot enough though that one would not want to stand in its way.
Now that the ice-sheets have retreated, the woolly wyvern’s range is restricted to the high mountains, where it lives amongst the snows, preying upon animals such as mountain goats, deer and so forth. The woolly wyvern’s thick hair and subcutaneous layers of fat mean that if it left the mountains, even to the foothills, it would seriously overheat - ones taken awya will be sickly and frequently die within a year. However, in a particularly cold winter, when ice and snow grips the land tightly, a woolly wyvern, driven from the heights by lack of prey, may venture down to the snow-covered lowlands to hunt for food. Its winged silhouette gliding across the sky, particularly in lands that may not have seen a wyvern in years, may strike terror in to the hearts of simple folk.
Woolly wyverns are territorial, each territory being inhabited by a mating pair. Woolly wyverns are monogamous and a female will typically lay one, occasionally two, eggs per year. Immature woolly wyverns, both male and female, typically have the worst territory (as there is only one of them to defend it) and the mortality rate is high amongst them. Territorial fights are fierce and bloody, though woolly wyverns will never use flame against another woolly wyvern - with their thick coat of hair they are particularly vulnerable to fire. They do not recognise ordinary wyverns as being their kin (though fortunately for both, their paths seldom cross as they inhabit radically different environments).
Interactions with humans
A woolly wyvern is almost always the top predator in its area; as such, they have little fear of humans (though they usually, but not always, avoid eating them). This is with good reason - few humans would be a match for one and, if in danger, the woolly wyvern can fly away. Certainly most mountain inhabitants and tribes know that it is best to give them a wide berth.
Other than their obvious prestige as trophies (should a hunter be able to kill one), the most valuable part of a woolly wyvern is its hide: when properly treated, this can be made in to a supple yet tough “leather”; as it is also covered in hair it combines light armour and warm clothing in one. Some archers are also known to prize the claws of a wyvern as arrows.