Generation Iv
Forewarn
Determines what moves the foe has.
Detailed Effect
When this Pokémon enters battle, it reveals the move with the highest base power known by any opposing Pokémon to all participating trainers. In the event of a tie, one is chosen at random. Moves without a listed base power are assigned one as follows: Power | Moves ----: | ----- 160 | One-hit KO moves: Fissure, Guillotine, Horn Drill, and Sheer Cold 120 | Counter moves: Counter, Metal Burst, and Mirror Coat 80 | Variable power or set damage: Crush Grip, Dragon Rage, Electro Ball, Endeavor, Final Gambit, Flail, Frustration, Grass Knot, Gyro Ball, Heat Crash, Heavy Slam, Hidden Power, Low Kick, Natural Gift, Night Shade, Psywave, Return, Reversal, Seismic Toss, Sonic Boom, Trump Card, and Wring Out 0 | Any such move not listed
Pokémon With Forewarn
6 canonical Pokémon forms can have this Ability.

Drowzee
Hypno
Jynx
Smoochum
Munna
Musharna