Penny's Vaporeon was revealed during the Intense Training battle against
Dot. Its Ability is Water Absorb. It won the battle against
Dot's Quaxwell as it targeted
Dot's Tinkaton and
Dot's Gholdengo on the sidelines with Muddy Water and
Dot's Quaxwell blocked the attack leaving it vulnerable.
Penny technically broke the rules of the match but she didn't cheat for the purpose of winning, she cheated because it was training for a serious battle against people like the Explorers that were likewise going to play dirty.
Dot needed to be prepared for this kind of behavior.