I give you a giant, flying, cave dolphin.
...a special bonus monster overhaul on the Yrthak. This one can keep all the stats the same, I think.
Here's the flavor text:
Ages ago, the aboleth began cultivating blind cave dolphins as slaves, food, and subjects for their bizarre experiments. The yrthak is the product of one such experiment: a breeding program to create a warbeast that would fight for the aboleth in the dry areas of the Underdark. The yrthak can still be found underground in particularly large caverns, but those that made it to the surface thrived there.
A yrthak has little in common with the cave dolphins from which it is descended. They are both blind, and they share a tubular body shape and sonar, but the yrthak are three to four times as large as a typical dolphin, averaging 20 feet long. Their fins have been altered to give them a 40-foot wingspan, and their dorsal fin has also been enlarged to aid them in flight. They have two rear claws in the place of a tail.
A yrthak senses sound and movement by means of echolocation and, straining itself, can emit a powerfully focused beams of sound usable as a devastating weapon. The creature is a yellowish-green color, with the wings and fin being more yellow and the head and body more green. The teeth are yellow.
Despite their intelligence, yrthaks do not speak. Some of those that live underground comprehend the language of aboleths.