The Coast Brothers were a group of three dognoses.
These 17th century privateers (members of the Beagle Boys' family line) appeared to be in possession of a letter of marque which legitimized their ownership of some treasure they had acquired. The straight-on pirate Morgan Duck robbed them of it and hid the treasure, which finally resurfaced in 2004 thanks to the investigations of Ludwig von Drake and his nephew Fethry; a trio of modern Beagle Boys tailed the Ducks, intent on stealing the treasure, as they were unaware of being its rightful owners.
Behind the scenes
The Coast Brothers were only ever mentioned in The Beagle Boys and the Last Pirate.
The Coast Brothers have a highly interesting background. They are, in the original Italian, known as the Fratelli della Costa, literally the "Brothers of the Coast", though "Coast Brothers" is also an acceptable, if literal, translation. The crux of the matter is that Fratelli della Costa is also the name of the informal organization of privateers (and later straight-up pirates) known in English as the Brethren of the Coast.
Thus, in the original, the reveal that the Coast Brothers of The Last Pirate were a genuine set of siblings, as opposed to a fraternity or brethren, was a sort of pun on the famous Fratelli — a play on words which, of course, was more or less lost in translation. It is somewhat notable that historically, Henry Morgan (on whom Morgan Duck is based) was a privateer and the informal founder of the Brethren of the Coast, as opposed to a ruthless pirate who antagonized them as Morgan Duck is depicted.
Interestingly, the English translation of Donald Duck and the Treasure Island has Yardleap again refer to pirates in general as "the Coast Brothers" in a translation of a reference to the Fratelli della Costa. (In this context, it is clear that he refers to pirates in general as opposed to the Coast Brothers in this story.)