We have a couple that we throw around. I call my boyfriend "P.R." which stands for Pet Rock. When we were first dating he was cutting down quite a bit and becoming very lean and it would have been like cuddling with a rock if he had continued at that rate. The other one I have for him is "Pookey" referring to Garfield's pet teddy bear. It was the first fictional name for a teddy bear that I could think of.
I somehow ended up with more. The first one I gave myself on accident, "2.0" since I am the second Mark he has dated and hence the improved upgraded version. Another one early on was "Mutty" referring to my sexual orientation. In the early stages I just thought I was bi-curious, so he said that I was little bit gay, bit straight, with some lesbian thrown in for good measure. The last one that he uses quite often is "Doomsday" which has been shortened to "Doomy" or "Do-me" if he's being an ass. This is in reference to the Superman character with the bones jutting out of him in every which way. I have very bony knuckles, elbows, shoulders, etc. One day while messing around, I jabbed him in the ribs and he felt a crack and thought I broke them. It hurt for him to cough for days after. Turns out I just severely bruised them, but I still had the potential to take down Superman.