Gemara

15b: Since half-damage is a fine, if a dog ate a sheep, or a cat ate a chicken, this is abnormal (a Toldah of Keren), so we do not collect it in Bavel.

This is only if it ate a large sheep or chicken. They normally eat small ones.

If the victim seized payment, we do not make him return it. If he asked to fix a time for Din in Eretz Yisrael, and the damager refuses, we excommunicate him.

In any case, we excommunicate him until he removes the damager.

(Beraisa - R. Noson): "Do not put blood in your house" forbids one to have a Kelev Ra (vicious dog) or a rickety ladder in his house.

46a (Mishnah - R. Elazar): A Mu'ad cannot be guarded. It must be killed.

(Abaye): He holds like R. Noson.

79b (Mishnah): One may raise dogs only if they are tied on a leash.

80a (Beraisa): If a convert inherited dogs, he need not sell them all immediately. He may sell them little by little.

(Beraisa - R. Shimon ben Elazar): We may raise Kufri (midget; some say, large hunting) dogs.

83a (Beraisa): One may raise a dog only if it is tied on a chain. One may raise a dog in a city near the border. He ties it during the day, and unties it at night.

(Beraisa - R. Eliezer ha'Gadol): Raising dogs is like raising pigs.

He teaches that one who does so is cursed.

Once, a pregnant woman went to bake in another's house. A dog in the house barked at her. The owner assured here 'don't worry. Its teeth were removed!'

The woman: The fetus already died.

Avodah Zarah 22b (Rav Yosef): A widow may not keep a dog (lest she have relations with it).

Rishonim

The Rif (6b) brings the Gemara on 15b.

Rif (20a): Even though the Halachah follows R. Noson, the Halachah does not follow R. Elazar.

The Rif and Rosh (29b,30a and 7:13,19) bring the Mishnah and Beraisos about raising dogs.

Rambam (Hilchos Nizkei Mamon 5:9): One may raise a dog only if it is tied on a chain. However, one may raise dogs in a border city. During the day he ties it, and at night he unties it. Chachamim forbade to raise dogs because their damage is great and common.

Rambam (10): If one inherited dogs, he need not sell them all immediately. He may sell them little by little.

Question (Rosh 1:20): It seems that 'in any case, we excommunicate him until he removes the damager' refers to a dog that ate large sheep, even once, for this is unusual and it is Keren. If we excommunicate even after one time, how does an animal become Mu'ad? It is difficult to say that he accepted excommunication mid'Rabanan.

Answer (Rosh): Attacking an animal bigger than itself is more unusual than regular Keren (therefore, we excommunicate). We do not excommunicate for a normal case of animal that gored, until it is Mu'ad. Alternatively, we do not excommunicate for an animal Mu'ad to gore animals, for it pays full damage.

Poskim

Shulchan Aruch (CM 409:3): One may raise a Kelev Ra only if it is tied on an iron chain. One may raise it in a border city and tie it during the day and untie it at night.

SMA (5): Since it is tied, people do not fear it, so women will not miscarry.

Hagahos Maimoniyos (Hilchos Rotze'ach 11:3): A widow may not keep a dog. This implies that others may have a dog. Only a Kelev Ra is forbidden.

Rebuttal (Yam Shel Shlomo 7:45): Perhaps only she may not raise a Kelev Kufri, i.e. it is small and does not bark, or it is big and does not damage

Shulchan Aruch (4): If one inherited dogs, he need not sell them all immediately. He may sell them little by little.

Prishah (7): The Beraisa discusses a convert, for it is uncommon that a Yisrael would transgress Chachamim's enactment, but if he did, the same applies (his heir need not sell them all immediately).

Rema: Some say that nowadays that we dwell amidst Nochrim, it is always permitted. See what people do! However, if it is a Kelev Ra and there is concern lest it damage people, one may raise it only if it is tied on an iron chain.

Gra (2): We who live among Nochrim are like people in border cities.

Shiltei ha'Giborim (1 DH b'Yom, on Mordechai Bava Kama 74): Why do people raise dogs nowadays? Perhaps we, who live among Nochrim, are in danger even during the day, like people in border cities at night.

Rebuttal (Yam Shel Shlomo): We see that we do not need more guarding during the day (that dogs will help for) than border cities. Therefore, also we should tie them during the day. Even if there were more concern nowadays, we cannot permit mortal danger, i.e. dogs in one's house, to save property! Chachamim permitted only at night, when people are not found. Rather, the Mishnah discusses only a Kelev Ra, like R. Noson said. It is unreasonable that one must chain a standard domestic dog. Even a domestic lion on a chain is permitted! Perhaps Stam dogs are called Ra, because they bark at anyone they do not recognize, even though they do not bite. Therefore, one with Yir'as Shamayim should not raise a dog other than a Kelev Kufri, which are recognizably for hunting or decoration. One should not raise a Stam dog. Even if it does not bite, it barks and can cause miscarriages.

Mishneh Halachos (5:296): It depends on the time, place and person. If a person is rich, thieves come with intent to kill or be killed. Surely this does not apply to an Oni! It also depends on his intent for having a dog.