Moles have a perfectly good name in Chinese (鼹鼠 yǎnshǔ), so why did Lín Shū invent a new character here? My theory is that Lín Shū, who famously did not speak English, asked the person interpreting the story what a 'mole' was, and got the reply 如鼠也 "it's like a rat", so he wrote the character as 如鼠.