I think it's because of the different colors of similar elements:
The horse and the rider both have horns, but they are different colors, and the rider's armor has spikes, which are also different colors. In any case, I think the shoulder pads and the lower part of the rider should be better aligned in color.
The fabric is especially noticeable. The horse is green, the weapon is red, and the rider is half green and half brown (if you look at the cloak, for example).
At the very least, I would definitely suggest changing the color of the fabric on the weapon. Otherwise, it would be better to make the entire rider either fully red or fully green.
*Of course, it's all up to you; I'm just sharing my opinion.

Added 5 minutes later
Another idea for the collection - if you can do it. You could try making a "ghostly" version of the horse, similar to the ghostly dragons, for upgrades. There are two ways to implement this:
1) Make a transparent model for its skeleton, preferably with an "aura" effect applied.
2) Simply make the skeleton darker and add an "aura" effect, so that the horse seems to be made of darkness or is a creature from the abyss, something from the other world. You could also give it colored eyes.