flounder, flatfish
鮃 centers on a specific type of flatfish: the flounder. It is a narrow zoological term without broad abstract extensions.
鮃 combines 魚 (fish) with 平, which likely contributes the sound and may relate to the flatness of the fish.
The fish radical 魚 beside 平 (flat) points to a flat fish: a flounder lying flat on the ocean floor.
For ヒョウ, imagine a flounder floating flat like a hyōtan (瓢箪, gourd) drifting in the water: hyō -> ヒョウ.