You can pick up the Hamster with Bernard (the kid with glasses in the present).
From the main lobby where you start the game, go upstairs (on the right). On the second floor, go all the way to the right and up the stairs again to reach the third floor. Enter the room on the right. You will go through some dialogue with Weird Ed Edison. Once this is over, pick up the Hamster in the glass cage on the right.
Once you have picked up the Hamster, open your inventory, click on the Hamster, and choose Hoagie or Laverne.
Bernard will say something along the lines of "you can't do that" (I forgot what it was, sorry!) while your achievement pops.
If you see a way to improve this guide or want further clarification, leave a comment! Thanks!