To get hog skins,
Google around until you find one you like (or use Octane's site as posted above while I was typing!). Then use
this program to apply them.
1) Open one of your Halo Maps in Halo Mapping Tools (that program I linked). Doesn't matter which one, they all apply the same. I picked Blood Gulch, personally.
2) Under "Bitmaps", go to "vehicles\warthog\bitmaps\warthog" (for rockethogs, it's "vehicles\rwarthog\bitmaps\rwarthog"), and save a copy of the current texture somewhere, like C:\Program Files\Halo\texturebackups\warthog.dds
3) Import the new one, and point it to the skin you want to use (take it out of any archive files first! [ZIP, RAR, BZ2 {I wish they were BZ2 compressed!}])
4) Save, and test by opening Halo and creating a server with default settings.