The Puckmaren is a little alien character who befriends David Freeman in Flight of the Navigator. He is from the planet Binpuka Minor, which was destroyed by a comet, and the last surviving member of his kind.
Max had collected the Puckmaren as one of his samples prior to Binpuka Minor's destruction, but naturally could not return him there afterwards. The Puckmaren did not know that his world was gone, and was angry that Max would not take him home, while Max could not bring himself to tell the Puckmaren that he now had no home to return to.
At the end of the film, the Puckmaren is discovered to have stowed away in David's backpack after he arrives back in 1978.