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:
Today I’m worrying over the Turris Mamilia, or the Tower of the Mamilii. Really there are only three pieces of evidence.
1) Passages in Festus. The reference is under the entry for the October Horse:
“October Horse” is the name of the horse which is annually sacrificed to Mars on the Campus Martius in the month of October. It is the right- hand horse of the winning pair in a chariot race. There used to be an intense struggle for its head between the inhabitants of the Subura and those of the Sacra Via: the latter hoping to affix it to the wall of the Regia, the former to the Mamilian Tower. And the tail of the same animal is conveyed to the Regia, with speed enough for the blood to drip from it to the hearth, for partaking in a divine service.
(Here’s a French translation.)
2) The fact that some members of the gens in the third century had the cognomen ‘Turrinus’. Refs can be found here and here.
3) An inscription, CIL 6.33837 = ILS 7242: “M. Octavius M. l. Attalus centurar[ius] a t. M.” where the t.M. is taken to be a reference to the Turris Mamilia as a topographical marker.
This is not a lot of evidence. Frankly its not absolutely clear that all three pieces of evidence refer to the same ‘tower’. What we make of it all pretty much depends on how one wants to think about the Festus passage. Is this tower an ancient and embedded part of the religious ritual? Is this proof of the Mamilii being part of the archaic Roman landscape on par with the ancient kings? Is some ancient power struggle between two claimants to the throne crystalized in this annual rite? Or was the tower just any old third-century landmark in an otherwise squalled, hot and dirty neighborhood? A point of focus in the district that was co-opted into the ritual contest for convenience sometime after its building presumably well after the origins of the religious festival itself?
None of the other sources on the October horse and the battle for its head emphasized the Mamilii in anyway, although we do hear about the Subura participants in this contest from Plutarch.