I wonder why Luffy, Zoro, and Usopp (and also Robin) were surprised when they saw the Dragon from Punk Hazard while the three of them have already seen Ryu, the Dragon that they met on Warship Island?
Their surprise stems from the fact that the Warship Island Arc is a filler arc. As One Piece wiki says:
It is the first story arc of the series not to be based on any content from the manga by Eiichiro Oda, making it the first filler arc.
Another reason might be the fact that they had encountered a dragon (if you count it as one) only once before, and didn't expect to meet one on Punk Hazard.
Because it's a filler arc.
In Canon story, they meet dragon for the first time in Punk Hazard. Having said that, it's a mythical creature which never existed in Pirate era, but it was an artificial one created by Genius Vegapunk.