The Loch Ness monster and Yetti predate the term Urban Legend, and are neither urban nor modern. They're just plain legends.

Urban Legends are made up stories that get passed around and some people believe, an oldie from the days of beehive hairdos is that a girl did her hair up in a "beehive" style, and a spider got in there, and she sprayed it with so much hairspray (so she didn't have to wash her hair) that the spider couldn't get out and laid its eggs in there, and she died from a million spider bites when they hatched.