I only heard of "such as its", but not "such was its".
What is the meaning of such was its below?
A horse like this had never been seen before — such was its splendor, its majesty, its strength
It's an inversion. Rearranged, it reads like this:
"Its splendor, its majesty, its strength was such [as had never been seen before].
(however, I would have used "were", because three qualities are mentioned.)