Mark 1:6 starts with
Et erat Joannes vestitus pilis cameli...
Which is translated as "John was clothed with camel's hairs...."
Why is it pilis instead of pilorum? Shouldn't pilis use genitive since it modifies vestitus?
Basically the genitive indicates that cameli is complementing the noun, specifying what kind of hairs are being talked about: those of a camel.
If pilorum was used instead, it would translate to of [the] hairs, and it would need to modify some other noun (e.g. colore pilorum cameli → with the color of a camel's hairs)
Now the ablative in pilis means the whole pilis cameli is an adjunct indicating the instrument used to accomplish the action (i.e., to get/be dresseed)