Machu's hair puffs out and stands all up on end when he's rumble-strutting around Yoshi. I guess he's trying to make himself look bigger, for dominance..?
Is your boy acting ok? A puffed up guinea pig, from what I've usually read, is a problem if they're lethargic or not taking any interest in what's going on around them. If he seems ok in himself it probably isn't anything to be massively concerned about.