In Avatar, the worlds of both man and spirit are never far apart. This was proven in the Winter Solstice two-parter where a being of the Spirit World terrorized a small town. Kidnapping some of the townsfolk and trapping them in the Spirit World
As Aang would find out, this spirit was called Hei Bai. And though at times it would be a gentle panda, when mad, it would transform into a monsterous creature. However, Hei Bai was not doing this out of malice, but out of sadness, for its home had been destroyed by the Fire Nation, and was in grief, lashing out on who he thought was responsible.
Aang corrected Hei Bai's view, and showed that the forest would grow back one day if he let it. The panda returned to normal, and returned the captured people. Hei Bai would later return to help Aang after he again ventured into the Spirit World.