Development
The earliest draft of Toy Story featured a slinky caterpillar who had never seen a child before. He was rescued from a toy collector and joins the main protagonists on their adventure before ending up at a kindergarten in the end. During rewrites, the character was changed to a dog, most likely to be more appealing.
Slinky was originally depicted as a long-suffering, abused right hand of Woody, and was also one of the most unpopular toys among Andy's collection. According to Woody, the only reason Slinky wasn't hauled off to Goodwill, much less is actually played with by Andy, was because of Woody. Eventually, after Woody not only attempted murder against Buzz Lightyear, as well as being severely insulted by Woody when he hesitantly spoke up against what Woody did, he turned against his former master by telling the other toys to throw him out the window just as he had done to Buzz. This concept, as well as Slinky's overall demeanor, was changed after the infamous Black Friday viewing of the film that resulted in it receiving an extensive redo.
Personality
Slinky acted very much so like an actual dog. He was nice, playful, liked to be pet, growled or barked at something he didn't like, hides when afraid, and was very loyal to Woody. Woody is shown to reciprocate his feelings and is often seen petting him, comforting him, and playing checkers with him. This sometimes leads to him looking like a kiss-up towards Woody (something the toys make fun of, especially in the original film). Due to his older-looking appearance, he is seen to be very relaxed and calm but admits in the third film, "I might be old, but I still got a spring in my stiff!".
