grey horse
騅 centers on a specific horse color: a grey horse, often with a dark mane and tail. The meaning is narrow and concrete, referring to this particular coat pattern.
騅 combines 馬 (horse) with 隹, which likely contributes the sound. The character specifically denotes a grey horse.
The left side 馬 means horse, and the right side 隹 looks like a bird. Imagine a grey horse with a mane as dark as a bird's feathers, making it easy to remember the color.
For スイ, picture a grey horse swimming (スイミング) across a river: swim -> スイ, and the horse's grey coat glistens in the water.