The present invention provides a novel collapsible pen that is used in combination with a widely available modular unit used in the shipping industry. The present invention is economical to use by virtue of the fact that the pen may be stored inexpensively in a minimum of space in a disassembled state, may be assembled quickly, and may be easily and readily disassembled to permit alternate uses of the modular freight unit. The present invention consists of a collapsible pen erected on a standardized rectangular platform which has at least one side open to the air. The pen is made up of modular panels. The panels are attached to the rectangular platform, and can be used alone or in combination with any side or end walls present on the rectangular platform to create an enclosure.
