This might be poetic licence, but I think it more likely that Beedle had only heard about Animagi, and never met one, for this is the only liberty that he takes with magical laws in the story.

这恐怕是属于诗人的大胆狂想了，但是我认为更有可能的是彼豆只听说过阿尼马格斯，并没有见过，因为故事中他只在这一点上歪曲了魔法规律。