variety of mole
鼢 refers to a specific variety of mole, a small burrowing mammal. The meaning is narrow and zoological, with no broader conceptual center beyond this animal type.
鼢 combines 鼠 (rat/mouse) with 分, which likely provides the sound. The character is a phono-semantic compound for a mole-like animal.
A mouse 鼠 that divides 分 the earth with its burrowing: a mole.
For フン, imagine a mole making a funny 'fun' sound as it digs: fun -> フン.