If you are in manual, you could do either. Everything will lighten though. If in A,S or P, changing the iso should not lighten or darken the picture, the exposure will readjust to keep it looking the same.
If you were in manual mode, lowering the shutter speed would lighten the horses (and everything else), same with raising the iso. But if you were in S, lowering the shutter speed would just change the aperture to compensate and keep the scene looking the same.
Assuming you were in A,S or P mode, you could have dialed in +1 or +2 exposure compensation OR you could have spot metered on the horses. Both of those would lighten everything. You could also lighten the shadows in processing. Shooting raw would give you more to work with.
I have my preview button programed for spot metering, so for a scene like that I can quickly spot meter.