What dates/area, rifle or bow? Your outfitter should be able to provide you with a gear list.
A comfortable day pack that can hold some extra layers/lunch will do, you likely wont need a big bag. Late Sept/Oct in the Yukon will likely be in the 32 to -4 degrees Fahrenheit range, depending on time of year and camp location. it hovered between 14 and 0 degrees for us on the last two moose hunts this past season.
Depending on your comfort level in cold temps, pack accordingly. I was running a couple layers of merino, with a synthetic + down insulation piece, down pants and Kuiu Yukon rain gear over top to break the wind. We had a couple blizzards to contend with and that worked out pretty good for me, but I was pretty lean after 3 months in the mountains so YMMV.
I would recommend buying high quality gloves & boots. The bulk of your day will likely be spent sitting and glassing, as well as sitting on the horse moving from camp and different glassing locations. I was wearing a pair of winter mountaineering boots that I run for sheep season and I would have payed a mint for a warm pair of pack boots, which I've since picked up. Once again, talk to your outfitter to get an idea of how the hunts typically go.
Take lots of pictures, glass as much as much as you can, and then keep glassing. If guides had a dollar for every minute clients have their binos down they would all be loaded.
I hunted north of Dawson the first time 2015 late September. Blizzard conditions the entire hunt and was not prepared with clothing. This hunt is early September rifle hunt eastern Yukon. Fly out from Whitehorse. Thanks for your reply. Just bought a exo mountain 2000, hope it is big enough.
Yeah that bag should work fine, I ran a Kuiu 1850 a little in moose season, and only switched back to my 7200 incase I was out day hunting without packhorses and needed to bring a cape back to work on it that night.