Was there ever any organization of thieves or pickpockets?

A recent Smithsonian article on Islam's medieval underworld might be of interest.

This broad group was known collectively as the Banu Sasan, and for half a dozen centuries its members might be encountered anywhere from Umayyad Spain to the Chinese border. Possessing their own tactics, tricks and slang, the Banu Sasan comprised a hidden counterpoint to the surface glories of Islam’s golden age. They were also celebrated as the subjects of a scattering of little-known but fascinating manuscripts that chronicled their lives, morals and methods..

A quick Google search unearthed more sources.

I'd also do some research into Tammany Hall particularly under the guidance of the infamous William "Boss" Tweed. While more of an example of political corruption than outright organized property crime, it is a very illustrative example of the interplay between criminals and public officials. In this particular example the line between the two became very blurry as a result of very dubious trades for political patronage of Tammany politicians from wealthy power brokers who sought influence, and the swelling immigrant population which sought material support and forms of patronage such as jobs or favors. This sort of interplay would be vital for any organized criminal group as corrupting government officials is necessary to either gain advantage or to at least turn a blind eye to their activities. Crackdowns on the rum-runners during Prohibition in the 1920's and the Italian Mafia in 1960's and 1970's, are examples of what happens when a corrupting relationship between law enforcement and organized criminal syndicates breaks down to the disfavor of the latter party.

For your purposes I would suggest looking into "The Gangs of New York". Both the 1927 novel by Herbert Asbury and the 2002 movie by Martin Scorsese based on said book. While I should warn the readers of this forum that both sources are of dubious historical accuracy, as a writer of fiction that is a lesser concern for you. That said, that source should provide some inspiration. It's good that you are making an effort to consult historical sources, and I urge you to follow up with the referenced sources on the Wikipedia articles I and other forum members have provided (what I consider to be a good practice whenever using Wikipedia).

If those sources are unclear, lacking or otherwise faulty this would definitely be the place to ask additional questions. I wish you the best of luck on your book!

Loki and Hermes are well-known gods of thievery.

As for saints, St. Nicolas is the patron of thieves.

As for clans, often there was some community of people that had any normal work forbidden by the society or some inner rules. Some opressed small nation could choose non-collaboration policy and crime remained their only way of living. Gipsies or something similar. So the clan often appeared as a ready thing, from the start, you are right here, IMHO.

Crime was always very well organized. The head thief in the late 18 cent. in Moscow became even the head of the police. Pirates in Rome times terrorized all the empire and had even their state. in Kilikia. (Also they had a state 1500 years later in Madagascar).

When a state is doing some its function badly, some crime organization will always appear to cover the function. And as our states are organized poorly mostly, the result is obvious.

Please, correct the question. The header and the body ask for different things.

You may want to check the cour des miracles (court of miracles) as a real life example of a "thieves guild". Clearly reading about the Mafia, Tong, and Yakuza should be compulsory as those are crime organisations. Look at your local law enforcement web site for further information on organised crime as well if you are looking for more modern organisations.