Interesting inteview with Phil on scifinow.com:

Here is an exerpt:

Was there anything you worked on that didn’t turn out quite as you hoped it would?

Phil Tippett:

“Everything, you know. You’re so anatomically focused on everything you can see how it would be a mistake. George is really good about that, with the walkers or just about anything, if you allowed him to he could find an editorial fix and just cut a scene where it was weak. Because the strength of the idea for the scenes of the walkers was so strong, that turned out good and successful, the Tautaun was more ‘hmmmmm’… it was a first stab because we were inventing something that later worked, but I don’t think those shots were as good as the walkers. The Tauntaun too was just a throwaway thing, it was like a horse so it didn’t really play, those were just shots, but if you have a strong theme you can get away with mediocre work [laughs] and the thing still works.”

http://www.scifinow.co.uk/interviews/star-wars-interview-phil-tippett/2/