These groundhogs are appearing on users' farms, just as regular groundhogs do. You'll need to arm your Groundhog Trap in the same way as if you were catching normal, disappearing groundhogs, but here once you capture either Phil or Fred, you'll see a new pop-up in your game, asking you whether or not you'd like to keep your new friend on your own Homestead, or if you'd like to share these friendly groundhogs with your friends by posting a news item on your wall.
Regardless of which choice you decide, this definitely gives the idea of building the Groundhog Trap more merit outside of simply finishing these goals. You might even want to build more than one to increase your trapping area to have a better chance at finding one or both of these new critters yourself.
Have you found any Frontier Freds or Phils while playing FrontierVille? Let us know in the comments.