When Mendelssohn was walking around in Scotland in 1829, he was inspired by the ruins of Holyrood Chapel in Edinburgh. The image of the place that once saw the crowning of Mary Queen of Scots, but now without roof and abandoned inspired the twenty years old composer to write his third symphony. He wanted to name it Scottish because of this inspiration.
When he finally finished what was actually his 5th symphony, it was no longer really Scottish. He wrote the finishing touches while he was visiting Berlin, and also a lot older: in 1842. Since it was still his third published symphony, it would get that number, and some people find in its dramatic tones the colors of the cold and wild north.