>>7539436They already had a fully rendered 'proper' model for Smaug, though - they even used it in theinitial theatrical release of 'An Unexpected Journey'. It wasn't about saving money - they spent MORE money to make a whole new model for him from scratch, then backtracked to edit over the preexisting footage. When asked about this, the reason given was that a wyvern was 'more believable', not that they were doing it because it was easier to design and animate.
