I've been rewatching TOS recently, and I noticed that Chekov's hair had a mind of its own for a while. I took a few screenshots, looked up the story behind it, and made this really quick for anyone like me who wasn't aware.
Roddenberry wanted Chekov to bring in the youth audience, so they gave him a wig to try to make him look like he's from the Monkees or the Beatles... absolutely hilarious.
That makes even more sense then! They didn't bring Chekov in until the second season, so they were probably trying to hastily compete with The Monkees.
They went a bit too far and at some points instead of creating a character for the Monkees ..... they were starting to create an actual monkey.