Horus hid his abilities.

"Your father is just a man, after all. He is brimming with fury, and burning with power. Such power. He is shining in your mindsight like a star. You had Him encased in a stifling, muffling, sense-depriving shell of pure, black immaterial force, but He has cracked that open, and now He burns a path towards you.The Vengeful Spirit creaks and shakes at the fury of His approach. He is so strong, so powerful, any being in the galaxy would shriek in eternal dread and hide in the depths of hell rather than face Him. But there is nothing in the galaxy like you any more. You no longer have to hide. You no longer have to conceal your power behind veils of secrecy and deceit. If He'd had an inkling of how strong you'd become, He would never have dared come to challenge you. So you wrapped all of yourself in un-when, in skeins of timelessness... your presence, your thoughts, your soul, your power."

-The End and The Death: Volume One.