Leonard was Lanny's twin. a congenital twin who was growing out of his side. But, Leonard was a bad twin (gasp!), for he would detach from sweet-hearted, weird, Lanny and perform homicidal acts, seeking "another brother." Lanny dies as does Leonard, presumably at the hands of the Conundrum, a circus geek who will eat ANYTHING.
"Humbug" was the first episode written by Darin Morgan, one of my personal favorites.
 

Spookyteacher's X-Trivia

3) Speaking of Darin Morgan, what role did he play on The X-Files during Season Two?
 

click above image or here for answer