Sahih Muslim, Book 004, Number 1032:
Abu Dharr reported: The Messenger of 'Allah (may peace be upon him) said: When any one of you stands for prayer and there is a thing before him equal to the back of the saddle that covers him and in case there is not before him (a thing) equal to the back of the saddle, his prayer would be cut off by (passing of an) ass, woman, and black Dog. I said: O Abu Dharr, what feature is there in a black dog which distinguish it from the red dog and the yellow dog? He said: O, son of my brother, I asked the Messenger of Allah (may peace be upon him) as you are asking me, and he said: The black dog is a devil.

Response

What we have to understand is dangerous animals like 'Coyotes', 'Jackals' and 'Hyenas' were all considered as Dogs back 1500 years ago in the Arab desert, Even today these dangerous Hyenas still live in large numbers in many areas where there are people living.

Many human beings have been attacked by Hyenas and they are very dangerous animals. For instance one source says

'When a hyena attacks a human, it usually begins by mauling the face first' (http://www.niletrip.com/dangers.html)

The 'black dogs' that Prophet Muhammad peace be upon him talked about were what we call today 'Hyenas' Some people call Hyenas 'black devils' even today as the Prophet peace be upon him called them 1500 years ago. Its just a description, Most hyenas are black.

They're also sometimes called 'Desert Wolfs'. So why shouldn't a person kill a Hyena, or a Coyote or a Jackal, or even a Wild Dog when you encounter them? There are also Africa's 'Wild Dogs' that hunt in packs also. These dogs are also black, and the Prophet might have also referred to them; the 'black devils'. These dogs eat their victims alove. Living in the desert was very dangerous, even more so back then. These animals are too dangerous to be around humans, and the YES, the Prophet was right when he ordered for them to be put to death

Another Muslim WritesThe killing of dogs is not something to be implemented today. It was only there for a particular point in time. The Prophet ordered the killing of these rabid dogs, for there were no rabbi shots back then. However, the question still remains. Why did the Prophet order the killing of dogs for a particular point in time? The wisdom for it is unknown. This is just a matter of faith and belief.

Alternative Response.... (http://www.livingislam.org/fiqhi/fiqha_e54.html)

Notes on the legal rulings based on this hadith and its meaning:

The passing of the above-mentioned in front of the person at prayer does not invalidate his prayer according to the vast majority of the Imams of Law, Salaf and Khalaf (early and late authorities) who concur that the meaning of "will cut off his prayer" is not literal but means: "will make his prayer imperfect" due to the distraction they will cause in people's hearts, while Imam Ahmad considered that only the passing of a black dog actually invalidates it. This was mentioned by Imam al-Nawawi in Sharh Sahih Muslim and al-Mubarakfuri in Tuhfa al-Ahwadhi.

Some of the narrations mention "the dog that bites indiscriminately" (al-kalb al-`aqur). Others: "The jet-black dog" (al-kalb al-aswad al-bahim). The gist of the reference seems to be that a wild-looking dog, or one known to be dangerous, has the greatest potential for distracting attention. The black dog is a devil either literally, i.e. a favored form for a devil to take among animals, or figuratively with respect to its negative aspects whatever these may be; and Allah knows best.

Here is another answer concerning dogs in general:

"Regarding killing dogs, the Hadith is authentic and quoted in numerous books of Ahaadith. In the pre-Islamic era, dogs were an inseparable part of the Arabs. The negative instincts of dogs have crept into their lives. Rasulullah came to reform them and in order to achieve that, he had to separate them from the inseparable, that is, dogs. He had to create a resentment of dogs in their hearts by emphasising in strongest terms like killing. He also ordered to wash the utensils seven times from which a dog drank.

"After the Sahaaba (Radhiallaahu 'anhu) understood the order and resented dogs, separated them from their daily activities, he said [= he made it understood], 'I have no problem with dogs'. He granted leniency in washing the utensils from 7 times to 3 times. After all, using hunting dogs is permissible and [dogs are] mentioned in the Qurân

(http://www.livingislam.org/fiqhi/fiqha_e54.html)