female rhinoceros
兕 refers specifically to a female rhinoceros. The meaning is narrow and concrete, with no broader conceptual cluster.
兕 is a pictograph of a rhinoceros, with the top 凹 suggesting the animal's thick hide or head and the bottom 儿 representing its legs. The exact historical development is uncertain.
The top 凹 looks like the tough, concave hide of a rhinoceros, and the bottom 儿 are its sturdy legs. Together they form a female rhinoceros.
For シ, imagine a female rhinoceros named Shi: Shi -> シ, and she is the only one in the herd.