My brother is a pen rider in a feedlot in Bovina, TX, owned by Cargill.
I'll relay some of his experiences:
The hours are long - on loading days they get there around 3:00 am and starting loading trucks. They need about 600 head to the packer before 6:00 am first shift.
The environment is dusty, and sometimes you can't see 5 feet in front of you when trailing cattle. Goggles are a must, or the dirt, dust, and manure will ruin your eyes.
When it rains, the pens can get like ice (manure + water = slick). His horse has went down a few times, but luckily he hasn't been hurt bad, just bruised up a little.
When your moving the finished cattle (1300 - 1400 lbs), a big horse helps; otherwise, they'll probably push, kick and throw you around. Don't ride ol 'Tiny' on loading day! ;-)
Horses:
I believe the feedlot will furnish horses to ride, but if you bring your own, you are alloted so much a month for the horse and they provide feed, board. You can only keep 2-3 on hand. Many pen riders take in outside horses for other people as a source of extra income. Always fun to get a bucking, crazy horse to ride pens with when Miss Betty Lou told you up front "He's really a good horse, he just needs some miles on him". :roll:
The feedlot however, will sure put some work on a horse. After opening 100's of gates, trailing cattle, and tracking through mud all day, many will tend to get a little cranky. So be ready to get slammed into the fence a few times here and there because your horse has had enough and is ready to call it a day.
Oh, and don't forget about those Texas panhandle winters- it can get mighty cold and windy, and there ain't no trees to hide behind.
All in all, I guess it ain't too bad a job. ;-)