be surprised
駭 centers on a sudden, startled reaction: being surprised or taken aback. The meaning is straightforward and does not branch into unrelated senses.
駭 combines 馬 (horse) with 亥, which likely contributes the sound. The idea of a horse being startled or shying away gives the meaning of surprise.
A horse (馬) suddenly rears up when startled by the boar-like shape (亥) beside it. The whole scene captures being surprised.
For ガイ, imagine a guy (ガイ) walking past a horse that gets so surprised it nearly kicks him: guy -> ガイ, and the horse is startled.