Among what trees did Zechariah see the man on a red horse standing?

Answer: Myrtle trees

In his first night vision Zechariah saw a man on a red horse standing among the myrtle trees in the hollow, with horses behind him.

“I looked out into the night and saw a man riding on a red horse. He was standing among the myrtle trees in the hollow, and behind him were red, sorrel, and white horses.”— Zechariah 1:8 (BSB)