Keanu is really just an Ent.
As evidence generally tends to point away from Keanu being a regular tree (see Keanu is really just a tree), some people might instead suggest that Keanu is not a normal tree, but an animated one – in other words an Ent, like Treebeard of Middle-Earth.
This would allow him to do things such as walk and talk (which normal trees don't do and Keanu has been seen to do), while at the same time retaining the essential tree nature that many people insist he has. At the same time, it would also explain the beard. However, once again this is not as valid an argument as might first appear, as I shall illustrate in the following paragraphs.
The evidence most strongly against Keanu being an Ent is his ability to do things fairly quickly. Ents are well-known for their extreme slowness in all areas of life, a trait which Keanu does not express.
Worth taking into consideration is the fact that Keanu acted in Speed, which is a film with a title most self-respecting Ents would find deeply offensive to their way of life. There is of course the possibility that Keanu just happened to be going through a rebellious phase at that point in time, and the fact that he turned down Speed II might indicate a repentful spirit as he matured and grew to better accept and embrace his Ent-ness.
However, Keanu nonetheless seems possessed with an innate love for speed. For starters, he has at least eight motorcycles, and riding them really fast is one of his favourite pasttimes. The more observant critics may point out the various accidents that this hobby of his has resulted in, one of which led to the rupturing of his spleen, and use this as evidence that Keanu was never meant to have such a quick-paced lifestyle; however, as blueberlin insightfully points out, "it's the desire to go fast that is indicative of his unEntness, not his inability to retain the wholeness of his spleen while going fast."
And as any hardcore fan of The Lord of the Rings can tell you, such a desire for speed is so un-Ent like that it can be used as conclusive proof that anyone who has such a desire – in this case Keanu – cannot be an Ent.
(with thanks to blueberlin)