Elderman - “Do I Have to Believe Evolution?”
Of course not. Just like you don't have to believe in gravity. However, both of these theories have voluminous amounts of observed evidence from thousands of different sources from all corners of the world that support them.
The example in your initial post about the Victorian fake at the Cardiff museum is interesting but it doesn't disprove or even cast any doubt upon the veracity of evolutionary theory. If you're using the above example to make a case for evolution being untrue then you might as well also claim that Leonardo Da Vinci didn't exist because there are some fake paintings of the Mona Lisa knocking about.
http://en.wikipedia.org/wiki/Ichthyosaurus