Saddled Rimestag’s effect checks each turn whether it applies during that turn. It doesn’t check only during the turn it enters the battlefield.

Saddled Rimestag’s effect applies even if the other creature entered the battlefield under your control before Saddled Rimestag entered the battlefield, and it continues to apply even if that creature leaves the battlefield.

Saddled Rimestag doesn’t get more than +2/+2 if you had more than one other creature enter the battlefield under your control during a turn.