Abstract: A horse mattress has a hollow body formed by a top bedding layer and a ground contacting base layer connected together to define an internal chamber, the top bedding layer being water resistant and sized to accommodate a horse; loose fill material within the internal chamber; a fill opening defined by the hollow body for adding and removing loose fill material to and from the internal chamber; and a reversible closure part for opening and closing the fill opening. A method includes supplying loose fill material into an internal chamber of a hollow body through a fill opening defined by the hollow body, the hollow body being water resistant; reversibly closing the fill opening; and placing the hollow body on a ground surface in a horse stall to provide a bed for a horse.