To this day when the Pokémon anime is referenced in other series, there are two things that one can expect to see: Pikachu, and Ash in his season 1 outfit, complete with the iconic red and white hat. Of course, Ash's hat was pretty significant to him in-universe as well, despite the fact that he would eventually go on to change it out for new hats. So where did Ash's hat come from, and what made it so important to him?
Even in the modern series, Ash is rarely seen without a hat on, but in the original seasons, Ash's hat was the focus a lot more often. Whenever Ash would concentrate in battle or catch a Pokémon, he would often turn the cap around, moving the bill out of his face. The stock shot of Ash turning his hat was used in dozens of episodes, and there was even an episode all about the hat, entitled "Primeape Goes Bananas."
In the episode, a Mankey steals Ash's hat and won't return it. Ash is distraught, and when Misty asks about the hat's significance, Ash explains that it's a Pokémon League Expo hat, one of only 100, and that he won it from a mail-in contest that he entered hundreds of times. The hat is effectively irreplaceable, so Ash is desperate to get it back. This does seem to be true compared to later anime episodes, as only one other character (the Ditto trainer Duplica) is ever shown to own the hat, and hers is only seen when she's impersonating Ash, suggesting it may be a knockoff made to look like his. The Mankey in the episode evolves and is eventually caught by Ash, allowing him to get his hat back and continue on his way. Ash would wear this hat until he made the decision to go to Hoenn, when he donned an entirely new outfit including a new hat and left the original cap in his room in Pallet Town.
Behind the Scenes on Ash's Hat
As it turns out, Ash's hat was always going to be a significant element of his character design. Ash's design is based on that of Red, the player character from Pokémon Red and Blue, and Red is similarly always shown wearing a red and white hat, although his is markedly different. The hat went through numerous redesigns to distinguish it from Red's, including a Pikachu-themed hat with a lightning bolt logo, a hat with a mark similar to the ones under his eyes, and various other triangular logos, according to concept art shown to the public. The final design bears a symbol (of the Pokémon League) that resembles the Japanese katakana character "mu," although some fans claim it is an L for "League." Ash's later hats, however, are drawn from the hats worn by protagonists beginning with Black and White, although they usually do still have some changes made to better fit Ash's design for the current era of the anime.
While Ash may have eventually left his original Pokémon costume behind, Ash's hat remains an iconic element of the series, even playing a role in Pikachu's Z-move. The hat likely still means a great deal to Ash too--perhaps he left it at home to avoid risking losing it again in Pokémon.