Your local Ciri representative is here to help you with your questions.
1.) Ciri's black mare is called Kelpie. Kelpie was left behind in the mortal realm after Ciri went to Aen Elle.
But she has a magical bracelet which can call Kelpie to her, so in theory Kelpie could make a reappearance.
The horse in that artwork is surely not Kelpie, but it could be Ihuarraquax.
In the books it is clearly stated that he can conceal his horn with magic and look like a normal horse.
My personal bet however is that she is simply riding a common steed.
2.) It's Ciri. I won't accept any other answer. Ciriously.
3.) No. I think it's just a normal horse.
Just because Ciri had Kelpie and Ihuarraquax, that doesn't mean that she needs to ride them the whole time.
Kelpie is probably somewhere else in the world, and Ihuarraquax in a whole other world to begin with.
So it makes sense to use a normal mount like Geralt does.
There you go,
Stay cirious !