bullock, fabulous monster
犍 centers on a castrated bull, a bullock. The 'fabulous monster' sense is a rare, separate mythological usage that does not connect to the bullock meaning.
犍 combines 牜 (cow) with 建, which likely contributes the sound. The character originally referred to a castrated bull, and the 'fabulous monster' meaning is a later, separate mythological extension.
A cow (牜) that has been 'built' (建) into a bullock: imagine a sturdy, castrated bull used for heavy work.
For ケン, picture Ken the bullock: a strong, castrated bull named Ken. Ken -> ケン.