Foundry Hornet

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.

Wotc — February 09, 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.

Wotc — February 09, 2017

If an effect causes Foundry Hornet to enter the battlefield with a +1/+1 counter on it, it satisfies its own triggered ability.

Wotc — February 09, 2017