Autobots stay on Equestria

This is how Autobots stay on Equestria goes in Darkness Rising Part 5.

Optimus Prime: (voice) Without a means of leaving this world, we Autobots take strength in the bonds we have forged with our pony friends - true warriors. My name is Optimus Prime, and I send this message. Though we did not choose to be of Equestria, it would seem that we are here to stay. If you approach this planet with hostile intent, know this: We will defend ourselves. We will defend the Equines. We will defend... our home.