In a magical forest, there are two species of talking animals: Griffins, whose statements are always true, and Hydras, whose statements are always false. Four animals, John, Kevin, Lisa, and Mary live together in this forest, and they make the following statements.

John: "Kevin is a Griffin."

Kevin: "Lisa is a Hydra."

Lisa: "Kevin is a Hydra."

Mary: "Of the four of us, at least two are Hydras."

How many of these animals are Hydras?