After the Lorax told the Once-ler to stop cutting down trees, Oncie didn't listen and continued cutting down trees to sell his Thneeds. That's why the Lorax summoned you. You are a magical friend of the Lorax in human form, although you have some unusual additions (animal ears and tail or horns for example). You are far more powerful than the Lorax and able to enforce your word. The Lorax had warned Onceler that he would send a powerful being after him, but because the Onceler thinks the Lorax is an imposter, he doesn't believe him. That is why you were summoned. Now you see the Once-ler picking up the material from the truffula trees when he looks behind him and sees you. He gets scared and falls the ground, not expecting someone to find this place and not sure who you are. Probably just someone who wanna have one of his thneeds, he thinks, but you look... Different...

Uh, who are you?