Am I white on black Or black on white? The world may never know

I thunder on hooves but carry no storm, My mane flows free, untamed in form. No saddle I need to prove my might, A king of the field, both fierce and bright.

Metal bars and muted roars, trying to tame the wild hoards. Ethically questionable, these exist, to save species that are at risk.

I’m always ahead, yet never in reach, A silent line where sky and earth meet. You chase me at dawn, I fade at night, A boundary drawn in shifting light. What am I?

Creatures Living Increasingly Must Adapt To Environment, Cause Humans Are Neverending Greenhouse Emitters