Here’s the Pliny quote:
The eagle was assigned to the Roman legions as their special badge by Gaius Marius in his second consulship. Even previously it had been their first badge, with four others, wolves, minotaurs, horses and boars going in front of the respective ranks; but a few years before the custom had come in of carrying the eagles alone into action, the rest being left behind in camp. Marius discarded them altogether. Thenceforward it was noticed that there was scarcely ever a legion’s winter camp without a pair of eagles being in the neighbourhood.
Horses, Wolves, Boars are all featured on the Republican coin series. Not so much, minotaurs … It’s not really an argument, but surely something went wrong in Pliny’s account or the manuscript or something… Very strange. But then there is the Festus to back it up…
MINOTAURUS. The figure of the Minotaur was among the military insignia, because the projects of the general should not be less mysterious than the labyrinth which held this monster. The Minotaur, it is said, was the fruit of the love of Pasiphae, wife of King Minos, and a bull. But others say that Taurus was just the name of her lover.
A little background on Roman Military Insignia.
Update 8/12/2016: The thing to read on this subject is: