Every inventor hopes that their next idea is the one that makes them a millionaire. For Gregg Miller the product that he made millions from was fake dog testicles.

He created the silicone implants so that dogs keep the same appearance downstairs after they're neutered. In the last 20 years, Miller has sold over 500,000 sets of Neuticles. Most of the implants are for dogs, but he once made a custom pair of watermelon-sized testicles for an elephant.

Miller says he bought a bloodhound puppy back in the 90s and refused to neuter him. Then one day his dog Buck ran off after smelling the scent of a female dog in heat.

He found Buck a few days later and decided it was time to neuter the dog. He asked the veterinarian to make some implants so his dog could "maintain his God-given natural look." The vet told him it was crazy at first, but then the two worked together to create a prototype.

The first pair of Neuticles went into a dog in 1995. Miller started advertising in dog magazines, did radio interviews, and built up a networks of veterinarians. Eventually he was getting 250-500 responses every day.

Read more at CNBC.