The Guardians of Ga'Hoole, Jareth the Goblin King, and Harry Potter's messenger owls have the power of creeping up on people. So do owls in real life, due to the design of their wings and feathers.
Owls are the bird of choice for the freaky supernatural beings. They look cool. They have the freaky-head-turn thing going for them. Mostly they evoke the otherworldly because they can move silently through the air; sneaking up on their prey from out of the darkness . . . RIGHTNOW!
The owl's wing is broad and rounded compared to that of other birds, which makes for less flapping, and a quieter ride through the air. That reduces some sound, but an owl is quiet even compared to other birds when they glide. It's more than just a difference in wing measurements. It's the fact that owls are fully upholstered in order to maintain silence.
The inside of music studios, concert halls and movie theaters is covered with blocks of fabric. These muffle sound and prevent echoing. An owl's hard surfaces, specifically its legs and feet, are covered with soft feathered that muffle noise.
It's the wing feathers that are really counterintuitive, though. Most people, when designing something that makes little noise, will make it as sleek and rounded as possible. That turns out to be a mistake. A sleek, streamlined thing won't have any extra parts banging off each other or creaking in the wind. As we've seen before, though, it will have turbulence. Turbulence is moving air and moving air makes noise.