From A Wiki of Ice and Fire
Rhaenys Targaryen riding Meraxes into battle
Meraxes was one of the great dragons used by Aegon the Conqueror and his sisters during the War of Conquest. The she-dragon was named for one of the old gods of Valyria. It was ridden by Rhaenys Targaryen.
Meraxes's was larger than Vhaghar but smaller than Balerion. Meraxes could swallow horses whole.  The colour of Meraxes's scales, horns, wing bones and spinal crest is unknown, as is the colour of her flame.
During the battle that became known as the Field of Fire, Meraxes, Balerion and Vhaghar took to the sky at the same time. Between the three they killed four thousand men, burning them alive. King Mern was killed, House Gardener dying with him. The army was broken and Aegon I was victorious.
Meraxes met her end in Dorne after an iron bolt went through her eye.  Along with eighteen other Targaryen dragon skulls Meraxes's skull used to hang on a wall in the Red Keep's throne room. After the War of the Usurper King Robert had Meraxes's skull removed and stored in a dank cellar along with the others.