When I was 2 years old, I was in love with horses... so much that my room was filled with nothing but them.

Anyway, looking for a cartoon from the '70 to the '90 because I don't know if it was that old, that involved unicorns, pegasus and horses.

A white pegasus with a horn and I think a blue unicorn? There were two girls involved... there weren't like my little ponny or anything. Normal girls and horses.

The two horses had diamonds in their breats.

I think it may be Princess Gwenevere and the Jewel Riders... That's the only thing that sounds remotely like that to me, it was a 90's show and it was awesome.