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As long as you had another creature enter the battlefield under your control this turn, Bellowing Elk has trample and indestructible.
Bellowing Elk’s effect applies even if the other creature entered the battlefield under your control before Bellowing Elk entered the battlefield, and it continues to apply even if that creature leaves the battlefield.
Bellowing Elk’s effect checks each turn whether it applies during that turn. It doesn’t check only during the turn it enters the battlefield.
