I am currently running the "Lure of the Lost" and have recently completed the Begginer game "Mountaintop Rescue". I am wanting to add in an encounter or two to the parties trip into Reles in the wilds somewhere, but am lost on what kind of creatures would be common on spintir and what would make a good and memorable encounter. Any ideas would be greatly appreciated.

Maybe an ice dragon of some sorts?

Spintir's native wildlife is almost all mammalian. Any non-mammalian species would likely beforeign and invasive. So keep that in mind. I believe that the beginners player guide lists a few names of critters that wander Spintir. The only one i remember is the Armoured Sloth though. Perhaps bumping into an enraged Dire version of such a beast could be memorable. (Maybe its mating season and alpha armoured sloths get super territorial or perhaps their is darkside influence.)