Byakhee

Described as "not altogether crows, nor moles, nor buzzards, nor ants, nor vampire bats, nor decomposed human beings," I've yet to actually see a Byakhee portrayed with all of those features. Because they were also said to be "tame, trained," they are generally treated as the mounts of the Mythos. So much so that they are often referred to as star-steeds.

In the RPG...

Despite being described as domesticated animals in The Festival, Byakhee in Call of Cthulhu have human-ranged INT and POW, so I think it's safe to assume that they are only subservient when affected by a binding spell. When ridden, they can fly their rider anywhere, including interstellar space. Though the Byakhee is immune to the dangers of space itself, this immunity does not transfer to the rider, who would therefore need their own manner of protection (such as Space-Mead). 

    As an enemy, the Byakhee's stats are not ridiculously high. It has only 2 armour, and an average of 14 hit-points. Its attack also only does an average of 7 damage, though if it manages to bite its victim, an average of 16 STR will be drained per round. It should be noted that in 7e, Byakhee have been slowed down a little. The old Malleus Monstrorum listed them as having 20 flight, while the rulebook and new Malleus has dropped this down to 16.

    One last thing I feel I should point out, is that the SIZ of a Byakhee is 5D6, meaning that 78% of them are as large as a human. This means that riding one would be akin to piggybacking on someone.

In a video game...

Because of what flight means for a game, I would be reluctant to include Byakhee as mounts. If it was a large open-world game, then that would be fine, but in any other game, including the ability to ride a Byakhee would probably only work as a means to (uncontrollably) move the player to a new location. As enemies, on the other hand, I think they would work just fine. They are certainly killable, and are sufficiently alien enough that seeing one crash through the roof or window would provide a nice scare for the player.

In a film...

I think Byakhee would make for great movie monsters. The story, The Festival, would itself make a pretty cool short film in my opinion, what with the Crawling Ones and Tulzscha in there as well.