It's actually a letter called þorn (capital is Þ) which was used in english until þe printing press got þere and þe printing press needed blocks wiþ letters on þem to print and þe blocks, along wiþ þe press were made in mainland europe where þey didn't use þ. Some people replacedit wiþ a y because þe handwritten version of þ looked like a y which is how we got ye olde