The Alien-like "Weeow Waoow Waooh" sounds in Jeff Wayne's The War of the Worlds Musical, That and the martian ULLA.
Prokofiev's Peter and the Wolf is composed solely of leitmotifs and narration, and is frequently used to introduce the concept to children.
The page quote is a reference to Peter and the Wolf. For the record, the oboe actually represented the duck.
Les Luthiers' Teresa y el Oso, which directly parodies Peter and the Wolf, does this to the extreme. The characters also have silly rhyming names in Spanish: "El Pajarillo Amarillo" (The Yellow Birdie), "La Mariposa Golosa" (The Sweet-toothed Butterfly), "El Molusco Pardusco" (The Brownish Molusk) and, most notably, "El Oso Libidinoso" (The Libidinous Bear).
As numerous references on this page make clear, Richard Wagner may be considered the KING of the Leitmotif; though he was not its inventor, he certainly made the most extensive, elaborate, and probably the most intelligent use of it throughout the canon of his works. His great Ring Cycle, for instance, consists almost entirely of a symphonic/dramatic development and interweaving of motives and themes and had a good 30 or 40 different ones in it!
Death metal band Bolt Thrower have the centrepiece song on most of their albums begin with a fade into the same riff, developing over the years. In its first iteration, World Eater, and in most since, it is also the outro to the song.
Coheed and Cambria have the "Time Skip" leitmotif appearing on several tracks across multiple albums, denoting...well, guess.
