Why do I have an extra whole on the underside of my penis?

Hypospadius? You would need to be evaluated, but it is possible that you have a hypospadius, where the urethra did not fully close all the way to the tip prior to birth. Sometimes the opening is hidden by the foreskin and if the doctor does not retract the foreskin, it may have never been diagnosed. Depending on the location, it may need to be repaired.
Possibly hypospadias. Condition where you can have a second opening on underside of penis. Men can very occasionally have a double urethra resulting in 2 holes.