As AddonPed, Squirrel Marston is fearless (won't flee) and will follow you around when whistled at.

To spawn Squirrel Marston as AddonPed, you'll need to have Rampage Trainer installed.
Add "squirrelmarston" to AddonPeds.txt located in: "Red Dead Redemption 2 > RampageFiles > Lists."
Then with Rampage running: Go "Spawner > Peds > AddonPeds."

If you don't want any of your in-game squirrels to be Squirrel Marstons, simply remove the file "a_c_squirrel_01.ymt".

- Mango