Riddle Poem

I am a best friend,
I reside inside an endogen plant,
I could be different sizes, even colors,
I am as mischievous as a child,
And i get excited when it comes to exercise.
What am I?

Answer:

"Dog with rose." Photo. 20 November 2008. Dog Blog. 2 March 2009. < http://stock-photos-windows-wallpapers.blogspot.com/2008/11/dogs-wallpaper-puppies-wallpaper-dogs.html>.