Bambi’s friend has that annoying habit of pounding his left hind foot on the ground (hence his name). He also a penchant for childish insults, but then, he’s not that much older than the fawn. (In Disney’s Bambi, Thumper was voiced by Peter Behn, who was four years old at the time.) He has to be reminded by his parents to heed the maxim now known as Thumper’s Law: “If you can’t say something nice, don’t say nothing at all.” Still, he takes an interest in mentoring the deer. He teaches him how to say a few words and even how to ice skate (though, as Thumper himself notes, Bambi on ice is “kinda wobbly”).
In a larger sense, Thumper helped Bambi by providing comic relief and by helping to establish a tradition in Disney cartoons of talking animal sidekicks who have more personality that the protagonists. As a result, he was rewarded with a whole “Disney Bunnies” line of merchandise, coloring books, and storybooks. In Who Framed Roger Rabbit, he’s said to be Roger’s uncle. So, not a bad career for a snarky little woodland creature.