Shadowbat

Shadowbats are the mounts of the Highborne guard near the Shores of the Well in Well of Eternity. After the Dragon Soul scene a few shadowbats appear and kill the Bronze dragons assisting the players.

The Shadow Bat was used as mounts by the Highborne loyal to Queen Azshara and Sargeras in the War of the Ancients. These mounts could attack their targets with their claws and teeth, and seemed to disappear and reappear at random like ghosts. Varo'then led his soldiers, who rode these bats, against the Dragons. It wasn't until Malfurion, who was riding atop a dragon, threw a cloud of seeds which stuck onto the bats, that they were finally revealed. This helped the Dragons and their riders to fight their enemy, although the shadow bat creatures were still elusive and in high numbers.

Speculation
They may have been felbats.