I had always chocked Garuda's appearance up to the fact that when a group of people create a god, they usually do so in their own image so they can relate to it. Ifrit takes on a bestial appearance, like the Amjala. Garuda appears birdish, like the Ixal. The Kobolds work within the earth, and venerate it above themselves. Others can be chalked up to tradition within the FF series. Bismarck has always been a whale, even if the Vanu look like giant mutant thrushes.

Plus, it's been established that the primals aren't the actual primals, but the physical manifestation of the mental image of the creature, given power by prayer.