This immense raptor unleashes a shrill cry as it bares its talons, each large enough to carry off a horse.
5e SRD
Rare, intelligent giant eagle with electric-blue coloration and crackling static electricity
As rare as giant eagles are, a Thunderbird is even more rare. Equally large as giant eagles, it is about 10 feet tall with a 20-foot wingspan and has coloration that includes various hues of electric-blue. The very air around a thunderbird crackles and sparks with static electricity. A thunderbird is intelligent and often speaks Common along with any local languages.
Basic Fantasy Field Guide Omnibus