The set of creatures affected by Foundry Hornetâ€™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 wonâ€™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.
If you donâ€™t control a creature with a +1/+1 counter as Foundry Hornet enters the battlefield, its ability doesnâ€™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.
If an effect causes Foundry Hornet to enter the battlefield with a +1/+1 counter on it, it satisfies its own triggered ability.