2020-06-23 : Abilities that trigger when counters are put on a creature trigger when a creature enters the battlefield with counters and when a player puts counters on a creature.

2020-06-23 : If you have two non-Hydra creatures with Wildwood Scourge's ability (perhaps because they were mutated using the Ikoria: Lair of Behemoths mutate ability), putting a +1/+1 counter on any non-Hydra creature you control will cause the two abilities to trigger each other indefinitely. If no player takes an action to break this loop, the game ends in a draw.

2020-06-23 : Wildwood Scourge gets just one +1/+1 counter, no matter how many counters were put on the non-Hydra creature.