Pink Pony can be played both on Linux and Windows and is both free and open source (see readme text).
Pony controls on the keyboard are:
Additional key controls:
If you have problems with more than two players on one keyboard, you can attach additional USB keyboards.
Gamepads are also supported: Joystick 1 controls Pony 1, etc. You steer the ponies with the left/right axis of the main stick and accelerate/decelerate with Buttons 1 and 2.
Pink Pony is hosted at github.com/ginkgo/pink-pony.
For more information, including compile instructions, credits and license information, please see the readme file.