When Foundry Hornet enters the battlefield, if you control a creature with a +1/+1 counter on it, creatures your opponents control get -1/-1 until end of turn.
2/9/2017 The set of creatures affected by Foundry Hornet’s triggered ability is determined as the ability resolves. Creatures opponents begin to control later in the turn and permanents opponents control that become creatures later in the turn won’t get -1/-1. The effect continues until end of turn even if you stop controlling a creature with a +1/+1 counter on it.
2/9/2017 If you don’t control a creature with a +1/+1 counter as Foundry Hornet enters the battlefield, its ability doesn’t trigger, even if you can give a +1/+1 counter to a creature right away. If you control no creatures with a +1/+1 counter as the ability resolves, nothing happens.
2/9/2017 If an effect causes Foundry Hornet to enter the battlefield with a +1/+1 counter on it, it satisfies its own triggered ability.