The reasons why a dog has a hard bag next to his urine may be due to the following reasons:

Urethra Polyps or tumors. Surgery is required to remove the growths or masses to restore urethral function. If necessary, urethral reconstruction surgery can be performed.

Stones block the urethra. Surgery is required to incise the urethra and remove the stones.

Inguinoscrotal hernia. Typically the protrusion varies in size, is tender to palpation, and requires surgical repair.

Since conditions vary, if you feel unwell, please consult an authoritative agency.