The emblem is first seen by Eckhart Venner from Treysa (a small city of Schwalmstadt). He was a notary and needs the emblem for signing his documents. It was first seen at 10th April 1555.
The creator of the emblem thought the name Fenner comes from Fähnrich (which means a flag bearer).
It is possible that this construction is correct for Fenners from Schwalm, but not for the swiss.
There are a lot of emblems, but this is the famous one: