I hadn't thought about it too much recently, but STB sent me a link to a Freepers article about the incident.
Here's a picture of the bunny in question, and I am not making this up.
Picture courtesy of the Jimmy Carter Library.
There is a large photo of Jimmy Carter and the rabbit at the Freepers page above.
Cecil Adams' Straight Dope says:
The rabbit incident happened on April 20 while Carter was taking a few days off in Plains, Georgia. He was fishing from a canoe in a pond when he spotted the fateful rabbit swimming toward him. It was never precisely determined what the rabbit's problem was. Carter, always trying to look at things from the other guy's point of view, later speculated that it was fleeing a predator. Whatever the case, it was definitely a troubled rabbit. "It was hissing menacingly, its teeth flashing and nostrils flared and making straight for the president," a press account said.Apparently it is a swamp rabbit. I have no idea if that makes this more freaky or less freaky.
The Secret Service having been caught flatfooted--I'll grant you an amphibious rabbit assault is a tough thing to defend against--the president did what he could to protect himself. Initially it was reported that he had hit the rabbit with his paddle. Realizing this would not play well with the Rabbit Lovers Guild, Carter later clarified that he had merely splashed water at the rabbit, which then swam off toward shore. A White House photographer, ever alert to history's pivotal moments, snapped a picture of the encounter for posterity.