A method for converting a horsebox between single and double occupancy has the separating wall (7) removable and replaced by two separating frames (7, 10) equally spaced from the centre line, enabling the single horse to stand centrally to improve the stability of the trailer. The separating frames are located into sockets in the floor of the trailer and are braced on either side wall by short spacers. The front and rear of the horse are secured by cross braces.