does anyone know?

Magnificent was possibly a Lipizzaner. Mostly because of his color, and the way he pranced. Lipizzans are "showy" horses, and have (from what I've seen) a distinct way of moving.

http://en.wikipedia.org/wiki/Lippizan

Hi,
I just saw the film. I would guess circus horse could be any breed cross. Just for guessing, Magnificent could be a lusitano cross, andalusian cross, or lippizan, maybe even a kladrubbey (spelling?). Probably Lippizan, just looks too much like one - has a lot of presence and elevation. Stunning horse! The appearance is superb for a black and white film. Wonder if this one happened to make an appearance in any other french films at the time?

According to the book "Beauty and the Beast: Diary of a Film" by Jean Cocteau:

"The horse, Aramis, arrived with his master from Paris at four o'clock. He looks like Rommel's horse which Montgomery rides. He's a white Arab and kneels down and rears like a wave crowned with foam. I'll keep his circus harness, for it is absolutely right for the style of a children's book. I've asked his owner to send his false tail."

Also, Mila Parely (Félicie) was injured when the horse reared while she was riding him in the fields around the farmhouse.
Josette Day (Belle) refused to ride Aramis, so a neighbor girl was her stand-in for the riding scenes.

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He was a white horse.

I like the white horse's glittery mane and he certainly lives up to his name of Magnificent. Also I noted Pegasus among the castle's sculptured figures that adds to the idea of the white horse being magical.