turtle
龞 centers on a turtle, specifically a soft-shelled turtle (suppon). It is a rare character used for this animal.
龞 combines 敝 (likely phonetic) with 龜 (turtle), forming a character for a type of turtle. The exact historical development is uncertain.
The top 敝 looks like a worn-out cloth, and the bottom 龜 is a turtle. Imagine a soft-shelled turtle hiding under a tattered cloth, blending in with the riverbed.
For ベツ, picture a vet (ベツ) examining a soft-shelled turtle: vet -> ベツ, and the turtle needs a check-up.