Continuity mistake: When Odie comes out on the porch to keep Garfield company, his collar and tags are visible; even more so when he attempts to get back in. But when the shot pulls away to show Odie alone on the steps (before running off after the scooter), his collar is gone. Part of the plot hinges on Jon trying to find Odie, who wasn't wearing his tags.
Answer: He's a cartoonist.