There are a total of 3 squirrels to find in Trials Fusion - Riders of the Rustlands DLC. After collecting a squirrel, you can quit that level. The achievement unlock upon collecting your last squirrel. Find the full list below.
] - Junkyard Funk
You must bail into the box underneath Checkpoint 1. To do this, go to checkpoint 2, ride up the ramp and let your rider go down the ramp backwards. Lean forward, then quickly back and bail shooting your character into the white box.
] - Rags to Riches
On Checkpoint 7 (or 6), bail out towards to wrecking ball that is hanging above the track. You'll need to bail towards it (high up), and then wait a few seconds. Now respawn your rider and do it again for the squirrel to trigger. (See video)
] - Gone to Waste
Get to Checkpoint 6, where there is a steep ramp jump over a gap. You'll need to bail your rider off this ramp, and fall into the gap between the ramp and the railing below, landing next to a big black container. Once you land, it will open to reveal the squirrel.