Beagler and NASA are both right. All the pens here the guys use 8 foot (2x4 welded wire) and bend it a foot at the bottom (to keep from digging) and bent a foot at the top (to keep from climbing over) All the pens around here have mostly coyotes in them, and only a couple even have a fox or two at best. Coyotes will run longer and better during the day, and have more stamina than a fox, but it takes more of a hound (in my opinion) to run a fox constatly when they are acting "sly" and turning and ducking every which way. Like NASA said they are used for training pups, or conditioning older hounds. There are escape areas for the coyotes, tubes behind electric fences, brush piles. Their own dens too. Coyotes are not to be caught, and if they do get bayed/caught the hound owners are to try to keep the coyote alive. Because they do cost money to buy.
And also like beagler said, with hound running areas on the "outside" continuly shrinking, these pens are becoming more common. I hope these few post have given you a little more insite. See ya, MOyotehunter