Using animals and humans as a wall:[1] Humans and animals are considered valid walls for a Sukkah, if they are 58 cm wide and 80 cm tall, and are not three Tefachim from the ground. However, animals are only valid if they are tied to the other walls of the Sukkah [in order so it does not run away]. On Yom Tov or Shabbos, it is forbidden to make a 1st, second, or third wall using an animal, and it is likewise forbidden to make such a wall using a human, if the human knows that he is being used as a wall. However, if he does not know that he is not being used as a wall, then it is valid to establish him as awall even on Shabbos and Yom Tov. It is permitted to make a 4th wall using a human or animal even on Shabbos. On Chol Hamoed, a human may be used as a wall even if he knows that this is the purpose of his stay. Vetzaruch Iyun if on Chol Hamoed the person himself can be used as a his own wall.
[1] See Michaber 630:11-12; Admur 362:12-17; M”B 630:52-57; Kaf Hachaim 630:74-87