Laudator Temporis Acti

Babrius 112 (tr. Ben Edwin Perry):

A mouse bit a bull. The bull felt the sting and went after the mouse; but the little fellow fled in safety to the inner part of his hole. The bull came to a stand and dug away into the walls with his horns until, having become weary, he sank down and went to sleep in front of the hole. Then the mouse peeped out, crept up, bit him again and retreated. The bull jumped up not knowing what to do next; whereupon the mouse squeaked to him this moral: "It's not always the big fellow who has the power; there are times when being small and humble has more force."

See here for the ancient Greek.

Avianus 31 (tr. J.W. and A.M. Duff):

They tell how once upon a time a little mouse on its wanderings ventured with its tiny teeth to attack a mighty ox. When its nibbling mouth finished biting, it thereupon hid safely in its winding hole. Though the ox made sullen threats with his huge neck, yet for all his anger he could not see that there lived an enemy for him to attack. Then the mouse dispersed the foe's threats with its cleverness, bantering the enraged ox with these words: "Because your parents transmitted strong limbs to you, it does not follow that they added efficiency to your strength. Learn, however, the self-reliance that our tiny mouths possess, and learn how our pigmy band does whatever it wants."

Ingentem fertur mus quondam parvus oberrans
  ausus ab exiguo laedere dente bovem.
verum ubi mordaci confecit vulnera rostro,
  tutus in anfractus conditur inde suos.
ille licet vasta torvum cervice minetur,
  non tamen iratus quem petat esse videt.
tunc indignantem iusto sermone fatigans
  distulit hostiles calliditate minas:
"non quia magna tibi tribuerunt membra parentes
  viribus effectum constituere tuis.
disce tamen brevibus quae sit fiducia rostris,
  et faciat quicquid parvula turba cupit."