Sprite of a raven from Subterranean Animism.
Ravens are a select few species of birds from the genus Corvus. The term raven does not exist in Japanese and are called crows (カラス karasu) and the differences between the names are all scientific. Although a raven is a type of crow, not all crows can be called ravens. Ravens are usually much larger than crows with different physical features, and are much more solitary than the crow which flock in masses. They are known to live much longer than crows as well.
Ravens in Touhou
Ravens can be found in Former Hell. The only noticeable difference between the ravens and the crows of Youkai Mountain during Mountain of Faith is that the raven has red eyes. The crows used by Aya Shameimaru in the fighting games had red eyes as well.
Hell ravens (地獄鴉 Jigokugarasu) have existed in the Hell of Blazing Fires for as long as it's been burning. Unlike normal ravens, they are born from the darkness of hell.
The Raven's Appearances
Ravens have appeared on stage 6 of Subterranean Animism as general enemies. These are likely to have some sort of relationship with Utsuho Reiuji. Just like their Mountain of Faith counterparts crows, they'll come flying onto the screen in big groups, and are non-trivial to deal with. It being stage 6, unlike crows, they'll stop flying and face towards the player, shooting bullet hell danmaku.