Look at this.
No, seriously. Testicle implants for dogs who've been neutered.
And in case you were wondering (from the FAQ):
Do Neuticles come in different models?
Neuticles are available in three models: NeuticleOriginals (rigid firmness) NeuticleNatural (natural firmness) and Neuticles UltraPLUS. Each are crafted from FDA medically-approved (for human use) materials- replicating the animals testicle in size, shape, weight and feel.
I want my dog's new balls to feel naturally firm, Doctor.