I was surprised that I'd never heard of "Thrashin" before, given that I was so into that culture at that time. What other movies out there are similar...meaning the personification of extreme sports in its infancy or documentaries on them?

Rad
Gleaming the Cube
Thrashin
Stoked: The Rise and Fall of Gator
Rising Son: The Legend of Skateboarder Christian Hosoi
(incidentally...Hosoi is a pastor in Cali now)
Klunkerz is a documentary of the birth of Mountain Biking
(similar attempt, but way different feel IMO)
Lords of Dogtown

The Search for Animal Chin is kind of a stretch b/c it was a spoof "fun movie" that was made by the Bones Brigade. Basically a skate vid that tried to have a plot. All of the skate vid's back then were fun, but not applicable IMO. If that were the case, we'd have to include every skate/bmx/MTB video ever made.