Eating other Pokémon
In the original Pokémon games and concept, it seems that most Pokémon were more animal-like. As such, wild Pokémon were originally portrayed as eating one another by some sources, in a very animal-like prey-predator system; for example, The Official Pokémon Handbook's entry for Pidgeot says that "When they hunt, Pidgeot fly on the surface of the water at top speed to catch unsuspecting Fish element prey like Magikarp." The Handbook also lists Golbat as drinking the blood of its enemies, not just sucking their "energy," as the Handbook refers to Leech Life in Zubat's entry.
The first time that eating other Pokémon is mentioned explicitly in the games is when a Team Rocket member mentions the SlowpokeTail's value as a delicacy.
However, this theme does not seem to have been explored in the anime or games; it seems the closest thing is found, other than SlowpokeTail, is attacks that steal an opponent's HP. This may be because of moral issues; as the anime and games progress, Pokémon seem to be growing more human, with personalities, human mannerisms, etc., and such behavior might border on cannibalism. However, some Pokédex entries still refer to Pokémon preying on each other, or at least sucking life force. This is seen in Haunter and Gengar, who in Pokémon Ranger, together with Gastly, lick the partner Pokémon until they vanish. The Pokédex entry in Ruby and Sapphire Versions also claims that Taillow feed on Wurmple. This is further explored in Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness, when Wurmple of Team Tasty thinks its partner, Swellow, will eventually eat it. (Swellow, however, seems mortified at the idea of eating her teammate). Cherubi's Pokémon Diamond Pokédex entry describes its appendage as being "very sweet and tasty," possibly confirming that Cherubi's ball can be safely consumed by humans. Its Platinum entry explains that Starly try to feed on Cherubi as well. Likewise, meat is often shown in the anime, but while it has never been shown to come from Pokémon, no other food source has been explained.