The letter Þ, which you come across these modern days in Icelandic (like in “Þingvellir”, the historical site of the Icelandic assembly), but almost nowhere else, is called Thorn. Or, more recursively, Þorn. And it’s pronounced like the “th” in the English word “thin.”
While the Þ isn’t used in today’s English, this was not always the case: in Middle English the word “the” was spelled “þe”. And the story goes that because early printing equipment lacked the letter Þ the compositors of the day substituted the letter Y when they needed to set it.
Which is how we’ve ended up with Ye olde as a device used to telegraph “old Englishness” — “Ye Olde Fudge Shoppe,” for example.