From Bulbapedia, the community-driven Pokémon encyclopedia.

Shadow Tag (Japanese: かげふみ Stepping on Shadows) is an ability introduced in Generation III that prevents the foe's escape. Run Away negates its effect. Similarly, as of Generation IV, if two Pokémon with Shadow Tag face one another, both may switch out at any time. See also Arena Trap. Only 2 Pokémon have this ability, both in the same evolutionary line.

Pokémon with Shadow Tag

Single ability

Wynaut	Wobbuffet

Dual ability

No Pokémon have Shadow Tag as one of their two abilities.