Is it possible that a monkey-like creature, in the mountains, could evolve to have no legs and a really strong set of fore-arms? I was thinking that this might be possible because of the climbing skills needed to navigate the environment.
I'm no biologist but I'll give some educated guesses. I'm gonna go with no.
First, if you look at any real monkey/monkey-ish creatures who mostly dwell in trees, not only do all of them have four limbs, most of their feet are "hands" that can grab. Not just that, most of them have tails that can also grab. I'm gonna guess that's because when you're living in trees for your entire life, you're gonna need all the limbs you have to support yourself, in order to move around safely.
That said, speaking of lack of rear limbs, bats have highly devolved rear limbs. If I recall correctly some of them have hooks for feet. Same goes with pterodactyl, because neither need to rely on limbs to travel - they can fly. So, if we're talking about monkey-like creatures with devolved legs, they'll have to be dwelling in highly vertical environment, like high cliffs covered with thick, strong vines all year long. And they'll have to be able to glide around like flying squirrels. And they'll probably need strong tails too.
The only way I could see something like that happening would be if a species like a cassowary bird were forced into trees from environmental pressures, but would only really have any chance if that niche were for some reason unoccupied.