horse's mane, fins
鬐 centers on long, flowing animal hair or appendages: specifically a horse's mane, and by extension, the fin-like structures on fish or other creatures that resemble a mane.
鬐 combines 髟 (hair) with 耆, which likely provides the sound and may relate to the idea of something long or aged, fitting the mane of an older horse. The exact historical development is uncertain.
The top 髟 means hair, and the bottom 耆 looks like an old man with a long beard. Picture an old horse with a flowing mane: 鬐 is that horse's mane.
For キ, imagine a key tangled in a horse's mane: key -> キ, and you have to comb through the mane to find it.