The vertical poles may be made of anything – even living creatures, though Rabbi Yosi prohibits this. A living creature can convey ritual impurity if used as the cover of a grave, but Rabbi Meir says it does not convey impurity. One may write a get (a bill of divorce) on a living creature, though Rabbi Yosi HaGlili declares such a get invalid.
If a caravan camped in a valley and they were surrounded by their animals’ gear (saddles, packs, etc.), they may carry within this enclosure so long as the gear forms a fence ten handbreadths high (about 30 inches) and that the gaps do not exceed the standing portions. A gap of about ten amos (about 15 feet) is permitted as a doorway; more than this is prohibited.