Marquis of Fantailler
A nobleman of Ankh-Morpork, or perhaps of Quirm, who got into a lot of fights - mostly because his name was the Marquis of Fantailler. Inventor of a list of rules on the manly art of pugilism, mostly concerning places you were not allowed to hit him. Following the Marquis of Fantailler's rules is a widely recognized way of committing suicide.
The character is a running joke, mentioned in several books.
The name may be a simple pun: the Marquis of Fantailler does not want to be hit in a fight (hence all the rules) and is thus a coward or is chicken; and fantail is a breed of domestic chicken - although fantail itself is a designation that can be applied to pigeons and other fancy birds. If this is the case, it explains the comment:
The Marquis of Fantailler got into many fights in his youth, most of them as a result of being known as the Marquis of Fantailler...
...which would presumably be incited by something like: Fantailler? What are you, a chicken?
Also: Roundworld boxing is based upon the rules of the Marquess of Queensberry, a name which may or may not have caused the same number of fights, and which probably inspired the character concept.
A later (by inference) Marquis1 stabbed his wife to death and tried to evade justice by fleeing to Fourecks, and disguising himself by the simple expedient of not using his title. In investigating the case, Sam Vimes ran up against the entrenched hostility of Ankh-Morpork's nobility who closed ranks and refused to talk, over and above expressing their collective indignation that a member of the nobility was being hounded as if he were a common criminal. Damn the thief-taker Vimes for getting ideas above his station, can't a chap commit one murder in peace? Besides, it was his wife's fault for having the crass and inconvenient bad taste to let herself die after only one stab! Vimes recollects this investigation in Snuff, while pondering the tendency of the nobility to hide behind privilege, and close ranks to protect each other.
The murder committed by the Marquis and his flight into self-imposed exile is very reminiscent of the Roundworld case of Lord Lucan. This member of the nobility tried to stab his wife to death one dark night. Incredibly, he got the wrong woman, and murdered his children's nanny, then fled in panic. The resultant closed-rank silence of the British nobility in protecting one of their own was not edifying and said a lot about their sense of ingrained privilege and of being above the law. The police claimed to have tried their hardest to crack the case, but may have been deterred by a sense of social expectations - i.e., you cannot haul in relatives of royalty and give them the same sort of robust questioning you wouldn't think twice about giving to an Irish bombing suspect or a West Indian or a striking miner. Comment was made about "It was only the nanny, for goodness sake!" and the British nobility made clear they knew perfectly well where Lucan was, but were not going to tell. In 2011, it is believed a criminal who fled justice in 1974 (and was covertly helped out by cash handouts from other nobles) died in exile, possibly in Australia or New Zealand.
1 In the Shires, the name is spelled with only one "l", likely just another failure of Morporkian spellynge. 'Snuff' also mentions a "fantailer hat".