Wendy's Spicy Chicken Sandwich
Wikipedia's article on Wendy's says it was added to the menu full-time in 1996, and was a promotional thing before then. There's no source given, unfortunately, but I found this 1996 shareholder report saying "This year we added two permanent additions to the menu -- the innovative Spicy Chicken Sandwich...", and on youtube you can find tons of copies of their ad marked as being from 1996.

I think it was first sold as a promotional thing in 1992, but I didn't find a really solid source yet. The best I've found is this ad for the new spicy chicken sandwitch marked as being from 1992.