A feedlot is an aggregation point for horses that are destined for slaughter. Similar to a kill pen in that regard. A feedlot can be USDA approved or a private feedlot, which might be owned by a kill buyer. USDA approved feedlots are regulated by the USDA and should adhere to a certain level of standards. Slaughter houses typically own their own feedlots.
A feedlot can be used to prepare a horse for slaughter whether that means improving the physical condition of the horse or providing a quarantine period to ensure there a no drugs in the horse's system. It is clear, however, that many horses go directly to slaughter from horse auctions and thus avoid this step.
There are licensed feedlots and they are listed with the USDA.
Some private feedlot owners provide access to rescues, but for the most part a feedlot is the last step before shipping to a slaughter house. Especially a feedlot that is owned by a slaughter house. Horses in those feedlots are considered "under quarantine".
Rescuing horses from a feedlot is itself a controversial issue within the horse rescue and anti slaughter community (see: AA's Investigations-Nickerson Feedlot). On the one hand it is the last option for a rescued horse, on the other hand by buying a horse from a feedlot the rescuer is doing business with a kill buyer which in turn makes his business more profitable and can enable him to purchase more horses for slaughter. When we consider that horse slaughter is a demand-based business, this issue becomes more accute.
List of known feedlots and horse slaughter houses they serve, ownership detail and whether they provide access to rescue.
Bel-Tex, TX, serves only the EU approved Beltex owned plant in Jerez (not Juarez), Mexico.
Morton, TX Frontier Meats
Stanley Packing, WA
Longmont, CO Operated by Charlie Carter one of the major kill buyers. This lot is large and can hold hundreds of horses. It is one miles from Interstate 25.
Fort Collins, CO Operated by Charlie Carter one of the major kill buyers. This lots is smaller and very close to the Centennial Livestock Auction. It is one mile from Interstate 25.
Stanwood, WA Owned by Florence Packing. Shipping to Canada. Likely Bouvry Exports.
Toppenish, WA Small feedlot serves Florence Packing's feedlot in Stanwood. Owned or contracted by Chuck Walker.
Los Lunas, NM Owner Dennis Chavez (Enumclaw Auction)
Phoenix, AZ, Owned by Dave Quinn. Associated with Pacific Livestock Auction.
Source for some of this: Animals Angels: US Investigations