尾 is in 尾巴 (tail), 结尾 (ending), 收尾 (to wrap up). It is also a classifier for fish: 一尾鱼.
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A colloquial reading yǐ also means 'tail', heard in casual 尾巴 and in 马尾 when it refers to horsehair as a material. The standard reading for nearly all words, including 马尾 as a ponytail, is wěi.
Outer indexing body radical 尸, a seated or reclining figure. Here it stands for the rear part of an animal's body, where the tail attaches. It anchors 尾 in the body family with 屋, 居, 屁, 尿.
Inside, 毛 is a pictograph of a tuft of fur or hair. Put fur at the back of the body and you get the picture of a tail, hair trailing behind. A compound ideograph: both pieces are semantic, with no phonetic involvement.
