Lump appearing on my dog's belly

Hi everyone, I hope you're all doing well. I'm a bit worried about my dog Paco, who is 9 years old. About 3 days ago, we noticed a lump that appeared right in the middle of his belly. There was nothing there before. He's eating, sleeping, and playing as usual, but the lump appeared quite suddenly. The vet thinks it might be a cyst or perhaps something cancerous, but they haven't done any proper tests yet. Have any of you dealt with this kind of lump on your dog before? What tests or steps did you take? Thanks so much for your help!