Has there ever been a cryptid proven to exist? A creature that general scientific consensus had regarded as being a myth for a long time, but finally proven to be real.
There are three confirmed animals on the list Konrad Rudolph provided:
(Though the turtle is most likely actually an already known species, just in an odd and not-usually-seen location.)
Two that are oddly (to me) not on the list are probably the most famous:
I can't work out whether the Platypus counts.
The story goes that when the first Platypus specimen was sent to England, it was perceived to be a fake (Ref) by the scientist, Dr George Shaw, who examined it with a pair of scissors in an effort to expose the fraud (Ref).
(The unreferenced web-sites I have checked differ in a few minor details: was it more than one naturalist who was doubtful? Can the scissor marks still be seen in the British Museum's specimen? I can't be sure.)
If one bloke from a museum in England can be counted as "general scientific consensus" (and perhaps, in 1799, he could!) then the platypus must count as a cryptid!
(There's a similar story about black swans, but I found even less evidence.)