When Sandsteppe Mastodon enters the battlefield, bolster 5. (Choose a creature with the least toughness among creatures you control and put five +1/+1 counters on it.)
11/24/2014 Bolster itself doesn’t target any creature, though some spells and abilities that bolster may have other effects that target creatures. For example, you could put counters on a creature with protection from white with Abzan Skycaptain’s bolster ability.
11/24/2014 You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.