If it's an enemy, walkers don't really have to attack at all if they feel like they won't survive it. I'll let lone enemy hogs drive by when I'm walking because it keeps everyone racing. But that's not what you're asking
If there are two lone enemy hogs driving past and one has the better hog, of course the walker can choose to go after the better one. It's the enemy's fault for not teaming up when it was safe. If you'd teamed up with the chain, you could have killed him and he never would have had a choice.
The rule about taking the first available hog is when there are empty ones available and there is no threat, so no reason to start fighting. If there had been an empty hog available and you in your hog and your teammate in their hog, then the enemy on foot should take the empty hog.
