What was unusual about a giant from Gath killed by David's nephew Jonathan?

Answer: Six fingers and six toes

The man had six fingers on each hand and six toes on each foot, twenty-four in all. He was descended from Rapha.

“And there was also a battle at Gath, where there was a man of great stature with six fingers on each hand and six toes on each foot—twenty-four in all. He too was descended from Rapha, and when he taunted Israel, Jonathan the son of David’s brother Shimei killed him.”— 2 Samuel 21:20-21 (BSB)