Answer:
The pet rock
Step-by-step explanation:
After hearing friends complaining about their pets and how demanding they felt the animals were, how messy they were and so on, Gary Dahl thought he had the perfect pet to solve all those issues: a rock.
He then decided to sell rocks as pet companions for people now wanting to be bothered with the usual troubles caused by living pets, since you don't have to take it out for a walk nor to change the litter box.