Zanam Djinn

The penalty is the entire point: build the deck this Djinn most belongs in and the body shrinks the moment it lands. Fill the board with blue permanents and blue becomes the most common color among all permanents (or merely ties for it), at which point the 5/6 drops to a 3/4 and tends to stay there. The counting clause is the genuinely clever part, because it surveys the whole battlefield rather than your half, and it can flip in either direction as both players develop their sides. Lands stay out of the tally: an Island is colorless, so flooding in blue mana sources does nothing to tip it. What looks like a mono-blue fatty is really a multicolor reward wearing a trap's clothing. In a manabase where blue is one slice among several other colors, the condition stays off and the full body earns its slot; the more committed you are to blue, the worse the creature gets. The early multicolor era liked to print color-pie commentary of exactly this shape, a stat penalty doing the soft work of pushing you off a one-color plan and toward a wider deck. It wants to be the lone blue card among many other colors, and it has sized itself accordingly.