I heard that the makers of the movie originally planned a sad ending. What happen in it?

Hmm, I am not aware of this, but then again maybe my memory escapes me on this. Perhaps someone else here can shed some light on the matter?

To the best of my knowlwdge, there were never plans of Bolt having a bad ending, although I think the OP refers to the infamous "Las Vegas scene" that was cut for being too tragic, but included as a deleted scene

Idk but the plans were a different plot and a different version of Bolt. I’m talking about Henry from American Dog, created by Chris Sanders who created Stitch. John Lasseter didn’t like Sanders’ idea and fired him when American Dog was already on development.