When did local toku heroes become a thing? What is the first of them like?
From the information I can find on Japanese websites (the English language info on Local Heroes is seriously lacking) the trend began in the mid-2000s with the arrival of Choujin Neiger in the city of Nikaho in Akita Prefecture.
Choujin Neiger was the brainchild of Tamotsu Ebina, a former Univeral Wrestling Federation wrestler and gym manager. Choujin Neiger’s true identity is Ken Akita and is the protector of agriculture and is designed to resemble a local oni called the namahage.
In the above picture, he is wielding his Buriko (Sandfish) Gun though his other weapons include the Kiritan (named after a traditional local food) swords:
His preferred method of transit is a vehicle named after a sauce for fish called the Shotstar, which transforms into his ride from a combine harvester.
Neiger transforms into his heroic form thanks to the mystical Gōshaku Jewel though he is not the only one to harness this power. There are several local heroes of Akita Prefecture, all of whom draw their powers from this gem. They include:
Neiger Geon the defender of forestry:
Aragemaru defender of fisheries:
Neiger Mai the defender of local industry:
Choujin Neiger became so popular that he spawned spin-off media including radio drams, manga, TV specials and a few songs about him, including one by the legendary Ichiro Mizuki!
Now, he might not be the very first Local Hero but from what I can find Chouin Neiger was the first to really get the trend started. Of course, a lot of this information is based on Japanese sources badly translated so it may not be entirely accurate.
Still, I hope this helps answer your question and introduce you to a new Local Hero!