Trials Fusion

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.
[]0:05] - 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.

[0:59] - 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)

[2:18] - 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.

Squirrel 1 - Junkyard Funk: Continue through the level until you reach the 1st checkpoint. From here you need to ride off the ramp in front of you, then bail out backwards so that you land inside the white metal box. There is a small square entrance underneath the ramp that you need to fit through. Using the button to grab and swing yourself into the hole may be useful. If you are struggling with this method, you can alternatively go to the 2nd checkpoint, ride up the ramp in front of you, then roll back down the ramp to gain speed, and then bail out backwards into the white box.

Squirrel 2 - Rags to Riches: Continue through the level until you reach the 6th checkpoint. From here you have to bail out towards the wrecking ball above you. No one is quite sure yet exactly how this squirrel works, as sometimes it appears, and sometimes it doesn't. It's possible that the squirrel may be a bit glitchy. When you do successfully activate the squirrel, he will appear on top of the wrecking ball, accompanied by a short cutscene.

Squirrel 3 - Gone to Waste: Continue through the level until you reach the 4th checkpoint. From here, accelerate up the ramp in front of you, then bail out downwards so that you fall through the gap. You will land in front of a large black container, wait here for a second and then the container will open, revealing the final squirrel.

Thanks to Hairy Cabbage and Evil of Sky for the info on the third squirrel.

Junkyard Funk: First Checkpoint, bailout into the big white box right underneath

Rags to Riches: Checkpoint 6, bailout on the upper ramp as high as the wreckingball in the background and just keep lying on the ground for a few seconds

Gone to Waste: Checkpoint 4, before you enter the great building, there is a small ramp, you see a black container below, drop down there and wait a few seconds