Metal tent pegs/pins/stakes certainly do exist during this time, we even have surviving Roman examples. However, they would not have been the most common. Iron, and later steel, can be very heavy and these would have been carried by foot soldiers or hauled by wagons in baggage trains. Either way, light weight is very important as is the ability to easily produce more if they are damaged or lost. Wood was the more common choice, therefore. Sometimes they were tipped in metal, but just plain oak or other hard wood stands up to the work well. There are very few surviving examples from the 18th century- a few originals belonging to George Washington's suite of marquee tents do survive, as do some early 19th c. replacements from when it was displayed by the family. While wood does sometimes split (from experience, mostly straight up from the notch), they are fast and easy to make from a resources that is available every where. It also means a wooden mallet (or the heel of your shoe) can drive the peg in easily. Wooden pegs were still being used as of WWII, though metal styles were more common by this time.