Why did Caesar keep Rex? Wouldn't he have had him destroyed as an abomination or something?

We are actually not sure when Caesar found Rex. He may have found him before he became known as Caesar so he couldn't part ways with him since he already had a bond. Also, he doesn't seem to care at all about Rex in the game. So maybe he let him go when Rex was "lost in battle."

Rex is a cyborg as opposed to a robot. He's a dog which has been enhanced with robotics. There's quite a difference between him and a full on combat robot.

Thus my use of the word abomination. I would think the legion would consider that type of thing to be a disgusting hybrid.

Were they close? I know Rex was formerly was part of CL (red bull paint), but haven't seen anything about The C-man himself.

I'm off to the wiki.

Edit: just read he was C-man's companion, but it didn't go into detail.