This is a really basic ATV I've built about 1 year ago. It is really simple and detailed too!
The back was inspired by the Lego Police Pursuit set:
Step 1: The Body.
Make these, than attach them all together.
Step 2: The Back
Step 3: The Front.
Make these and attach them together.
Step 4: Your Done!
Here is your complete simple LEGO ATV!