I can not stress enough that you must go read Immortal by Chris Clarke.  It is about his dog, Zeke, who is old and finally succumbing to the travesties of age.  In his usual way, Chris strikes unmercifully at the heart with his rumination about a friend who will probably not live another year.  This is powerful stuff.

I have been his world forever. My life stretches on past his for perhaps twice his allotted time or more, our time together the center of it. I am immortal to him and yet I find no comfort in it, nor in the prospect of a tidier house next year, an extra hour in the mornings. Were my lifespan flesh I would give him half. His expectant stare has always worked on me. I never managed to teach him not to beg.