If pine trees were bad for horses I'd be screwed. There's nothing wrong with pine trees and horses. Mine like to lay in the big piles of pine needles underneath them. You do have to be careful about fire around pine trees...very flammable. Learned through experience.
The only issues with pine trees are pine sap, which can be hard to get out of winter coats (and summer coats, LOL.) And they do seem to attract lightning more so than other trees (likely due to their root system.) Of course me being in Florida could account for the lightning hitting the pine trees more frequently, but still.