grey horse
駰 centers on a specific horse color: a grey horse. The meaning is narrow and concrete, with no clear abstract extensions.
駰 combines 馬 (horse) with 因, which likely contributes the sound. The character was created to name a specific horse color.
A horse (馬) that is the color of a grey, misty enclosure (因): picture a grey horse standing inside a foggy pen.
For イン, imagine an inn where all the horses are grey: inn -> イン, and every horse in the stable is a grey horse.