I don't know about racism amongst animals.
I have some questions first:
How many fjords did you encounter this with? 2, 5, 30?
Does she know these fjords or are they strangers to her?
Did you ever encounter her acting the same way against any non-fjord horse?
For now I can only give you an example that might be somewhat like this:
My mother's dog, Roxy was attacked by a shepherd when she was a baby. Since then she is frantic about them. Of course her behaviour against shepherds makes her fear come true every time.
As Gina is a crossbreed german/belgian shepherd it is almost impossible to have holidays together as we do not want to leave Gina and my mother does not want to leave Roxy.
They just can't be together while they are perfect with any other dog. (In Roxy's case, except shepherds of course.)
In case of the horses;
I have also noticed that for company and friendship horses often (but not always) seek out their own breed. Don Jamie is most of the time intrigued and prone to befriend any Hiberico that comes to our grounds, male or female.
Next to that he interacts happilly with all baroque breeds, quarter horses and arabes.
The same goes for the kinsky boys. Both he and the kinsky's seem to have no interest in warm blood horses. They act as though they are air!
Even the women... in heat!
(Imagine their reactions...
sorry gal, they go for the Rubinesque small ones... best gain some weight and make yourself small, maybe crawl somewhat... and grow more mane...
The same goes for large draught horses, simply does not interest them.
I also have the idea that stallions seem to like mares most with the same colour as their mother.
More I can't tell you... maybe the others have some experiences to share?