Wow. This blog is written by a Spanish man, Fernando Martinez Larranaga, and is full of great images of crests, barding, trappings and other heraldic items.
The following images were from the post labelled, 'Book of the brotherhood of the Knights of Santiago', also known as 'Libro de los Caballeros de Santiago'. http://www.facsimilefinder.com/export/pdf/446 lists the book as being created in 1388 and completed by 1600. You can order a copy and see previews of this and other manuscripts for sale, here.
A mixed colour feather plume on a horses brow, matching his rider's hat trim. There is also another horse with a more modest feathered plume.
I think those are bells on the caparison, along the spine.
It is the feathers I am noticing, here.
