Have you ever looked off to the side while moving quickly and noticed some kind of creature following at the same pace? Then you've managed to find a Runalong!
Runalongs are very fluid in appearance, their features shift a lot in a manner similar to certain Beings. However, they do have certain aspects that remain constant.
They are very tall, about eye-level with an adult male human riding a fully grown horse (Their exact height can vary wildly, but this is a good average)
They move on at least 4 legs, often more.
They are covered in either fur or feathers on their main body. This appears to only be different between individual Runalongs, as I've never seen them switch.
They are pitch black, and light seems to have a prismatic effect when reflected off their fur/feathers.
They have wings, and can fly with them.
They appear to be vaguely cat-like in body structure, but their ears and tail change to different species often.
Runalongs are rather peaceful, as they tend to stick to a distance unless called. (I found a melody they seem to like) Their fur/feathers are very soft.
They appear to be rather protective of their young, but will let you interact with them if they trust you. (I'm fairly certain I might of been adopted by one, seeing as it keeps following me.)
I suspect they either have a connection to displacer beasts or Barrier (Confirmed, there's a Being ) as they have the ability to slip in and out of reality as they please. They tend to only follow for the duration of a trip and vanish when the travel is over.
All in all, Runalongs are very pleasant creatures. They simply want to move with others.