Thirty white horses on a red hill, First they champ, Then they stamp, Then they stand still.

“Horses”

While the riddle mentions 'thirty white horses', it is a metaphorical description rather than a literal answer. The 'horses' represent teeth, not actual animals.

“Marbles”

Marbles may be round and can be described in colors, but they do not fit the action of 'champing' or 'stamping', which relates to the movements of teeth during chewing.

“Snow”

Snow is white and can cover a red hill, but it does not relate to the actions described in the riddle, such as 'champing' or 'stamping', which are specific to the process of chewing.