Nath's Buffoon from Lorwyn is a boggart in the employ of Nath, the imperious elf. The art shows the shadow of his elf master.
The card Sleeper Agent looks like a human woman, but her shadow shows that she's actually a minion of Phyrexia.
Check out the art of Summoner's Pact. We don't see the creature summoned by the green-aligned mage, but we certainly see its shadow. Usul has called a big one.
Time Spiral's Divine Congregation shows a circular time rift. Through the center of it, we see the shadows of a group of aven who have gathered together there, to join in the congregation (the card is a suspend riff on Congregate).