Hiya!
If you want random, meaningless encounters for your campaign, well, just pick any monsters with a CR around 9 to 12. However, like most DM's (I hope!), you take pride in your campaign world. So...
What kind of mountains? Sparse, granite covered ones, or green-covered lush ones? Temperate zone? Tropical? Arid? What towns are near the mountain's base? Are they hodge-podge of random locations folks just kinda settled in from wherever? Is there a lot of civilized activity in the area like mining, trapping, or something else? Is the area the mountains found in, in some kind of dispute with nearby countries/cities/people? The list of questions you need to answer or at least think about is nigh-endless.
Once you have at least some of those answered you can then compile a list of races/monsters that are likely found in the area. This gives you room to add in the "unique" monsters and stuff. For example, if you end up with a bunch of 'low powered' monsters like goblins/kobolds/orcs, you can then place a "crazed Stone Giant that calls himself The Lord of Stone" who lives in a particular valley/pass area and allows nobody to pass without providing him with a gift or tribute. The small-fry monsters avoid him, but stay near him for protection from the unique 'winged owlbears' that nest and mate in the higher-up mountain areas. The woodsmen and miners in the area advise to avoid walking in the open to avoid a horrible death from either the owlbears, or, if too close, a large boulder landing on your head (thrown from a crazed Stone Giant).
Now that you have that, you can whip up some random encounter charts, make some notes that pop into your imagination during game, and all this info will be "repeatable" 6 months from now when the players have their PC's wander back down this mountain range on their way back. They will remember the things you placed, and it will all seem that much more real. Additionally, they can "plan" for the trip...like making sure to get a small barrel of pickled herring that the Lord of Stone seemed to be asking for last time.
Sorry I wasn't much help in the "here are some monster encounters" department...but if you think of the world FIRST, and then think of the PC's SECOND, you will be able to "wing it" no matter where your PC's end up. Your players will love you for it because they won't feel they 'wasted' something (spell selection, item acquirements, class choices, etc)...because they will be able to say things like "I guess we travel along the edge of the Benneth Mountains again. At least we can stop in at Springstart and see that cute serving wench, Ilya again! Oh, and don't forget to pack the barrel of pickled herring! A couple of extra healing kits too...those kobold traps aren't that deadly, but man do they sting!"
^_^
Paul L. Ming
