They always have a different color tint and increased size (although some bosses might keep their size or even shrink).
Champion monsters have more health, possess larger shots, and deal a full heart of damage instead of half. They always drop a Pick Up when killed.
Champion bosses come with more drastic differences: Aside from increased damage, some might even posses new abilities (the champion Larry Jr. gaining the ability to spit shots, for example) or are otherwise more threatening (Monstro might appear with a duplicate, and Peep can start with both eyes removed). Faster movement (like Gemini or Chub) is also common, or a change in spawning behavior (Gurdy might just spawn instead of shooting, and Chub might also spawn Spities or Boils instead of Chargers).
Champion forms of bosses will always drop a heart upon defeat when found outside a boss room or an extra heart when found inside a boss room.
- When leaving a room and reentering before defeating all the monsters or bosses, the chance of a monster or boss being a champion form is randomized again.
- It is possible to meet two champions with the exact same color in the same room.
- Edmund McMillen's blog talking about "alternate" and "special" enemy forms.
- Certain monsters, like the Evil Twin, are considered Champions by the game in The Womb, and will drop a Pick Up upon death.