What magical dog was fated to always catch its prey?

The magical dog fated to always catch its prey was Laelaps.

Laelaps was a supernatural hound whose defining power was that nothing it pursued could escape. In one tradition, Zeus gave the dog to Europa, and it later passed through the hands of King Minos before reaching Procris. Other versions connect the hound directly with Artemis and Procris.

The most famous episode begins when Procris’s husband, Cephalus, used Laelaps to hunt the Teumessian fox. That fox was destined never to be caught, creating an impossible clash between two absolute prophecies: Laelaps had to catch whatever it chased, while the fox could never be captured.

Zeus resolved the contradiction by turning both animals to stone. The myth also links them with the night sky: Laelaps became associated with Canis Major, while the Teumessian fox was connected in later tradition with Canis Minor. Laelaps is not Cerberus, the underworld watchdog, or Orthrus, the two-headed hound of Geryon.