Creatures are resistant to elements if the stat blocks say they are resistant to the elements.
Guess I wasn't clear enough with my questions/ statement of facts...
The winter witch archtype says:
"Winter witches must choose a familiar that is native to the frozen north, even when they themselves operate in other regions. Traditionally, this limits winter witch familiar choices to bat, cat, fox, hawk, owl, rat, raven, or weasel."
However none of these creatures stat blocks SAY they are resistant to cold, so unless I'm missing something in the rules (refer to my first post), apparently every winter witches familiars have to make checks to not take environmental damage constantly... So either the winter witches are always down one first level spell to keep their familiars alive or they replace familiars quite often.