Gemara

51b (Mishnah) Question: What may animals go out with on Shabbos?

Answer: A camel may go out with a halter (a collar around the head). A Na'akah (white/female camel; alternatively, a mule) may go out with an iron nose-ring. A donkey of Luv (Libya, or Mitzrayim) may go out with iron bit. A horse may go out with a Shir (a collar with a strap or rope attached);

All Ba'alei Shir (animals that normally go out with a Shir) may go out with a Shir and are pulled by a Shir [on Shabbos].

(Rav Yehudah citing Shmuel) Question: People asked Rebbi whether or not an animal may go out with Shemirah that is permitted for a different animal.

Surely, a Na'akah with a halter is insufficient Shemirah, so it is like carrying a load;

Rather, we ask about a [regular] camel with an iron nose-ring. Since a halter would suffice, it is like a load;

Or, perhaps excessive Shemirah is not considered a load.

Answer #1 (R. Yishmael b'Rebbi Yosi): My father taught that four animals go out with a halter -- a horse, mule, camel and donkey.

Inference: A camel may go out with a halter, but not with an iron nose-ring!

Rejection: No, we infer that a camel may go out with a halter, but a white camel may not.

Answer #2 (Beraisa): A donkey of Luv or a camel may go out with a halter.

(Answer #3): Tana'im argue about whether or not excessive Shemirah is considered a load;

(Beraisa): A Chayah (wild animal) may not go out with a collar;

Chananyah permits a collar, and anything that guards it.

They argue about a cat. The first Tana holds that since a small rope suffices for it, a collar is a load. Chananyah holds that excessive Shemirah is not considered a load.

(Rav Huna bar Chiya citing Shmuel): The Halachah follows Chananyah.

Question (Levi the son of Rav Huna bar Chiya): May an evil donkey go out with an iron bit?

Answer (Rabah bar Rav Huna): Your father taught that the Halachah follows Chananyah. (It is permitted even if it would be excessive Shemirah.)

(Mishnah): A cow may not go out with a strap between its horns.

(R. Yirmiyah bar Aba): Rav and Shmuel argue about this. One forbids whether it is for Noy (decoration) or for Shemirah (to hold it by the strap. This is excessive, for one can lead a cow without holding it);

The other forbids if it is for Noy, but permits if it is for Shemirah.

(Rav Yosef): Surely, Shmuel permits for Shemirah, for Rav Huna bar Chiya said in Shmuel's name that the Halachah follows Chananyah! (Excessive Shemirah is not a load.)

Objection (Abaye): Just the contrary, Shmuel forbids even for Shemirah! He said that people asked Rebbi about this, and R. Yosi taught that four animals go out with a halter -- a horse, mule, camel and donkey;

Inference: A camel may go out with a halter, but not with a nose-ring!

Answer (Rav Yosef): We reject that teaching (it must be mistaken) due to Rav Huna's teaching.

Question: Perhaps we should reject Rav Huna's teaching and accept Rav Yehudah's!

Answer: We learn from what other Amora'im said in the name of Rav and Shmuel:

(R. Chiya bar Aba citing Rav): A cow may not go out with a strap between its horns, whether it is for Noy or for Shemirah.

(R. Chiya bar Avin citing Shmuel): It is forbidden if it is for Noy. It is permitted if it is for Shemirah.

Rishonim

Rif and Rosh (24a and 5:1): A Beraisa teaches that a donkey of Luv or a camel may go out with a halter. We infer that it one may not go out with an iron nose-ring, for even excessive Shemirah is considered a load. The Halachah follows Rav. Three later Amora'im answered for Rav. Also, the Halachah follows Rav against Shmuel in Isurim.

Rosh (ibid.): We permit donkeys of Luv with an iron bit, even though we find that a (regular) halter suffices. This is because for them, it is not such excessive Shemirah. For a regular donkey, it is excessive Shemirah, and it is considered a load.

Rambam (Hilchos Shabbos 20:8): One may pull an animal by its bit or Resen (bridle) to Reshus ha'Rabim, if a Resen is proper for it, e.g. a horse with a Shir, a camel with a halter, a Na'akah with an iron nose-ring, and a dog with a collar. If one took out an animal with a bit that does not guard it, e.g. he tied a rope in a horse's mouth, or a bit that is not needed, for it is guarded with something smaller, e.g. a donkey with a horse's Shir or a cat with a collar, this is a load. Any excessive Shemirah or insufficient Shemirah is a load.

Poskim

Shulchan Aruch (OC 305:1): An animal may go out with something in which it is guarded, but not in something that is insufficient, or is excessive Shemirah, and it is like a load. Anything for Noy and not for Shemirah is forbidden.

Beis Yosef (DH Kol): Rashi explains that one is commanded that his animal rest on Shabbos. Something that guards an animal is like a Tachshit (ornament). It is normal. It is not considered a load. Something that does not guard an animal is considered a load. We concluded that Rav forbids a cow with a ribbon even if it is for Shemirah. The Rif, Rambam, Rosh and Tosfos rule like Rav.

Beis Yosef (DH v'Chasav): R. Yerucham, Semag and Sefer ha'Terumah say that excessive Shemirah is a load only if it is a great Shemirah. A medium Shemirah is permitted, even if a small Shemirah suffices. One cannot precisely gauge the exact amount of Shemirah needed without any excess. The Mishnah says that a horse goes out with a Shir, and a Beraisa permits with a halter!

Magen Avraham (1): Noy is forbidden if it is not normal (Rashi, Ran). The Bach rules like Tosfos, who forbid [even if it is normal].

Mishnah Berurah (1): Something needed for Shemirah is a Tachshit. It is normal, like clothing on a person.

Shulchan Aruch (ibid.): Therefore, a Na'akah may go out with an iron nose-ring inserted through a hole in its nose. A donkey of Luv may go out with iron bit, a camel with a halter, and a mule, donkey or horse with a halter or bridle, but not with both of them. One may wind the rope of the halter around the neck and go out with it. One may move the halter and put it on the animal, as long as he does not lean on it.

Taz (2): The Mishnah permits a donkey of Luv with an iron bit. The Gemara says that a regular donkey may not, for it is excessive Shemirah. The Shulchan Aruch rules like this in Sa'if 3. If so, why does the Shulchan Aruch here permit a donkey with Resen, which is Ritzu, i.e. an iron bit?! We must say that Ritzu is not an iron bit. However, the Beis Yosef equates them. We must distinguish them. It seems that Resen is the general category. The Beis Yosef means that also Priga is called Resen, but it is not the iron bit mentioned above. Surely a camel is forbidden with an iron bit. If not, the Beraisa that teaches that a donkey of Luv or camel goes out with a halter, it should have said also an iron bit! Rather, it is excessive Shemirah for a camel. The Mishnah connotes like this. Even though the Mordechai permits a horse even with an iron bit, for it is not such excessive Shemirah, one cannot say so about a camel.

Magen Avraham (2): The text of the Shulchan Aruch should say like the Tur says, that a horse may go out with a Resen. A donkey may not go out with a Resen, which is an iron bit (Bach). The Bach permits a camel with an iron bit. There is no need to say that this is correct.

Gra (DH u'Pered, DH v'Sus): The Shulchan Aruch should read "a camel may go out with a halter. The same applies to a mule or donkey. A horse may go with a halter or bridle, but not with both of them." This is like the Gemara, Tosfos (52a DH Amar) and Tur. See the Magen Avraham.

Note: The Gra connotes that also the Magen Avraham explains like this. Eshel Avraham explicitly says so. Machatzis ha'Shekel explains the Magen Avraham to say that "mule or donkey" is a printing mistake, and deletes it from the text.

Gra (DH Aval): Tosfos (52a DH Amar) says that it may not go out with both of them, for the latter is [unnecessary, and therefore] a load.

Mishnah Berurah (10): If it is wild, one may go out with both of them (Mordechai).

Mishnah Berurah (11): One may wind around the neck only for animals that go out with a halter, if it is long. A cow may not go out with a halter, so one may not wind a rope around its neck, since it is guarded without this.

Mishnah Berurah (12): Rashi and the Ran connote that it is permitted even if it is wound so tightly that one cannot stick his hand between the halter and the animal's neck, which is a convenient way to lead it. Even so, it is permitted, for it is Noy. Any Noy that it normally wears during the week is permitted on Shabbos. However, Tosfos and R. Yerucham forbid Noy in every case. Here we discuss when it is loose, and it is considered Shemirah, for he can hold it by the rope. The Bach is stringent like Tosfos.