Red gyarados + dragonair = OMG shiny dratini/magicarp (like you know, or not, girl=FI, boy=FF (FF means nothing) if gyarados=FF the breeding will be magicarp.....
they can breed because they are both "Dragon Egged Species" click [link=http://www.serebii.net]HERE[/link]for more information.
the chance of having a shiny this way is 64 to 1 i guess) With the dratini you could make a shiny salamence, horsea.... etc