A “Kill Pen” is an area of an equine auction site that typically holds horses intended to be sold specifically for slaughter.
People who buy horses for slaughter attend these auctions and head for the kill pens to bid on the animals by the pound. If their bids are successful, the horses are loaded onto a tractor trailer for transport to Canada or Mexico. It’s a long, ugly trip.
There are two primary Auction Sites that have kill pens on the east coast. One is in New Jersey, the other is in Pennsylvania.