Cosmo's temporary animal friends are animals that Cosmo got interested in, named, and tried to befriend in Sleepover and Over. All of them except Jimmy were killed
Carl the Ant
Carl was an ant that Cosmo found and named in Timmy's treehouse. Later when Timmy tried to tell his godparents to hide due to his friends arriving, he accidentally shone his flashlight at Cosmo's eye, hurting it and causing Cosmo to throw up the magnifying glass he was using to observe Carl. The magnifying glass then crushed Carl and Cosmo kept crying about him even after Timmy's sleepover ended in his friends becoming enemies. At the end of the episode Cosmo cries about Carl again and says "I want my records back!".
Cindy the Bee
Cindy was a bee that appeared near the entrance to the junkyard while Timmy and his fairies tried to come up with an idea to be at both his friends' sleepovers. After Cosmo named Cindy and went after her, she was eaten by a bird shortly after. After seeing this, Cosmo mourned that even though he only knew her for 12 seconds, he'll miss Cindy forever briefly before eventually forgetting who she was while Timmy and his fairies are at A.J's sleepover.
Jimmy the Cat
Jimmy is a cat in the junkyard that Cosmo tried to befriend, only to scare him by barking at him. This made Cosmo and Wanda forget the signal on Timmy's plan to get his friends back together (as they realized their sleepovers stunk and Timmy got inspiration from the Major Geek Channel that was on A.J.'s TV). Later, Jimmy was hit by cars after running into the street (and apparently by a horse and a train, given the sound effects), however he survived since he had nine lives and he only lost three in the accidents. He then shredded Cosmo into strips and then stole his dip.