I guess it kinda depends on the symbolism and themes of the faction. For example, for my Sun faction I'm using a Phoenix because they both have a strong theme of rebirth (the sun being "reborn" every morning and the phoenix being reborn upon death). I like to select units based on metaphorical connections like that. If you're looking for a holy flying unit, maybe a griffin would fit the bill? Lions and eagles are both often considered to be noble and majestic, and the griffin combines the two. The Ziz also crossed my mind, since it's the only giant bird I can think of with significance in a religious context. A chimera could be cool, but I don't know how well it fits.