Giant Tree Devourer

When the first settlers arrived on this new planet, they wondered quite quickly why trees would grow so spare in certain regions, even though it was a perfect location for trees growing.

Soon they discovered the "Giant Tree Devourer", a creature as big as a mountain.

When at first all lived in fear of such a big creature, it was soon noticed that this creature actually only feeds on plants - trees to be more specific. So the tree mystery was solved.
Scientists examined the humongous creature and the following quite interest facts were discovered:
The creature needed to eat trees to survive, making it move constantly to find new feeding places - the huge head actually worked as a sail and so the creatures wandered where ever the wind would take them. At the time of a storm, this huge sail could be folded together, giving less attack points for the furious wind.
Only eating trees wouldn't give enough energy for such a huge body, especially since the next forest can be a long way off. It was found that the creature had a symbiosis with the various plants growing on it and also inherited some of those features - the huge wind shield was a huge leaf, absorbing the energy of the sun.