noun
front runner; front-running horse
Horse racing term for a horse that runs near the front of the field, often setting the pace.
あの先行馬はスタートから先頭に立った。
That front runner took the lead from the start.
先行馬がペースを握ると、後続は苦しくなる。
When the front runner controls the pace, the chasers have a hard time.
差し馬 is a horse that comes from behind in the stretch, contrasting with the forward-placed 先行馬.
Compound of 先行 (せんこう, 'preceding, going ahead') and 馬 (ば, 'horse').