To get Poe Ratcho you first need to unlock Garlic, which is unlocked by finding floor chickens.
After unlocking Garlic it will begin to appear in the pool of choices from leveling up and randomly in treasure chests. Select Garlic 7 times in a run (or find it in treasure chests) to get it to level 7. Once you complete that run, you’ll get this achievement, and Poe Ratcho will be purchasable on the character select screen.