There are actually fish in the sea of Galilee which pick the fertilized eggs off the sea floor and hold them in their mouths until they hatch. Some use this to prove that this was not a miracle. But the response to that is how did Jesus know that that particular fish had a coin in its mouth, that Peter would catch the right one and how did he know its denomination or amount. It seems that Jesus had not been at the Passover - perhaps up in Gentile territory. Therefore he wasn't around to pay the tax.
Royalty goes free.
Sons are exempt. They don't pay taxes. This is a religious issue - temple tax. All the citizens of Israel. The Son of God.