I have seen a few GLORIA EXERCITVS coins from Arles described as having a bull on the top of the standards. On her excel lists for this type, Dane even says this-- "The horned bulls' heads on the banners could refer to Legio VIII Avgvsta (whose emblem was a bull), stationed near Arles at this time."
There is not a bull, or bull's horns, on the standard. It is a ring, just a plain ring at the top. Below are two coins of this type. The first one is not completely clear and maybe there is some room for interpretation, hence the confusion; but the second picture clearly shows the rings and nothing but the rings. In the first picture, the rings even look thinner at the top and thicker at the bottom, which is similar to horns, except the rings join together or pass through what looks like a globe.
In 2008, I posted in a topic on Ancients.Info about this, but it seems eveyone wanted to believe in bulls...head
http://www.ancients.info/forums/showthread.php?t=1727&highlight=arles+bull