Monsters your opponent controls cannot attack the turn they are Summoned.
Rulings
Individual Card FAQs
The effect of “Big-Tusked Mammoth” includes those monsters Summoned this turn before “Big-Tusked Mammoth” was face-up on the field. For example, if your opponent attacks with a monster that was Summoned this turn, and you activate “Call of the Haunted” to Special Summon “Big-Tusked Mammoth” from the Graveyard, this causes a replay, and your opponent now cannot attack with the monster that was Summoned this turn.
If “Big-Tusked Mammoth” is Set on the field, and is attacked by a monster that was Summoned this turn, the attack resolves normally.
Monsters flipped face-up with an effect like “Book of Taiyou” are not affected by “Big-Tusked Mammoth” and can attack that turn.
If “Big-Tusked Mammoth” is on the field, and a monster that was Summoned this turn is removed from play temporarily and returns to the field the same turn, it still cannot attack.
EARTH|
Level 5|[ Beast / Effect ]
ATK 2000|
DEF 1000