Hey everyone just wondering if a runner can become a rival ?
It can. I have a level 4 FNIX one. Have not killed it yet though. It is quite rare to get one.
SPOILER for FNIX Rising
One of the side missions you actually fight a proto runner rival.
Ok cool i was sure it was a glitch or something
No, not a glitch but really rare.
btw I had two of them and both were transformed to rivals after they had killed me
I had an Apoc runner rival some time ago - I left him to roam for quite some time due to the rarity of it, but when I finally killed him, he only dropped a few rounds of ammo. Guess the loot pool on these isn’t up to the badassitude of their names. 
I also had an apoc runner spawn, his name was Wolverine , I too kept him around for awhile kind of like a pet. Until one day he snuck up on me and snapped at me , I put him down quick. Also dropped nothing but bullets