I feel like human nipples just because it would be way more convenient. Getting the whole extra torso under the horse body seems like a pain. I need someone to draw both options now so we can discuss.
Eta: wait wait, are centaur babies human baby sized with proportionately small horse parts, or are they baby horse sized with a bigger human parts to match? That could change my answer.
I feel like they should be colt sized, because in most depictions centaurs don't seem to actually have accessible human genitals, so they should be gestating in the horse body and not the human body.
But they’re not literally half human half horse, they’re fully centaur.
So there’s no human genitalia or horse body, only centaur genitalia and centaur body. Where the centaur genitalia is on the centaur body could be anywhere, ex. horse based locale dong but human based mammaries.
They have no genitalia at all, they can conceive from kissing, there saliva acts as sperm. Males and females can lay eggs from this. Often both lay an egg from the same kiss and these are considered siblings. The gender is decided by the gender of the egg layer.
Centaur salivia when inserted in a functional human vagina can rarely produce a weird human-centaur hybrid but those babies cannot survive past a few weeks.
I am just making this up as i go but Its literally impossible to proof me wrong so…