Derrick Wyatt, Art Director/Lead Character Designer for Transformers Animated, has posted some images on his blog showing some of the design concepts that he used for Transformers Animated Prowl prior to settling on the final design.  As well as the images Derrick reveals that it has already been decided that Prowl would be a motorbike before he came on-board with the series, that he had originally had him coloured more like the black and white G1 version, but since Ratchet was already going to be primarily white, and the ninja aspect of the new Prowl's character was going to be played up, they decided to go with the black, gold and tan theme.  Derrick also mentions that Eric Siebenaler from Hasbro and Alex Kubalski from Takara requested that they move Prowl's wheels from his shoulders to his legs as it helped blend his design with "a really amazing transformation idea" that Alex had been working on for a motorcycle Transformer.

If you look closely on the image you will see that it is dated 2003, giving you some indication of when the work on Transformers Animated first began.