Heeheehee...you only think
you "might get out on a trail every two weeks", just wait until you have your own truck and trailer! You'll never be home anymore!
And while now
you only have one horse and don't haul often and you could
get by with a little two horse and a half ton truck, I would suggest you go with a 3/4 ton and a little bigger trailer. And I would highly recommend making it a 4x4 as well.
My main reasoning, you are in some pretty serious "windy mountain roads" country. A heavier rig will hold the road better, control the motion of your trailer better, will handle the hill climbs and bottom of the hill stops much, much better, etc. And that 4 wheel drive is a life saver when you find a washed out road and need to do a bit of off-roading to get by or even just get turned around!
This is purely from personal experience. I started out my hauling career with a nearly new 1972, 2 wheel drive 1/2 ton Chevy and a W-W 2 horse steel trailer. I was in the cascade range of Oregon at the time, and my poor little truck had to work way harder than it should have, and there were a couple times I very nearly didn't get stopped before wrecking at the bottom of some really steep winding roads. Granted, I was new to hauling, and didn't plan ahead enough on those instances. Had I been paying attention there probably wouldn't have been a problem, but had I had a little heavier rig, there would have been no problem even NOT paying attention.
Although I live in pretty flat country now and haul with a heavy 1/2 ton van, factory set up to pull a trailer, because I inherited it, I still recommend a 3/4 ton to anyone who is in the process of buying a rig. There's just so much peace of mind to get in and feel safe
in your rig.