A Frostmaul Giant from Frostwhisper Gorge in Winterspring.
These guys have a pretty nasty reputation, especially if you have to enter their domain for a quest. At 60 I didn't find them all that tough, and I soloed the gorge a couple of times before I got bored and moved on.
This clip shows the Treants attacking the Giant first. Despite the whining that goes on in the official WOW forums, I find the Treants useful. They are a good first attack for an enemy that you need to wear down a bit before they turn on you. Not quite the tank that a pet can be, but pretty good nevertheless. Where they are really useful is when you get ambushed by multiple mobs. Setting the Treants loose is a good way to occupy one or two of the enemies while you prepare a defense. The negative side is, like the mechanical yeti or the Greench, they are not controllable. Therefore if you turn them loose in a crowded place, like an instance, you may regret it.
You will notice, if the mob hits really hard, they go down rather quickly. The first Treant in this video goes down after four or five hits, and the second follows soon after.