Photos showed the all-black animal roaming the White House North Lawn with what appeared to be a pink flamingo dog toy.
Bo, a male of the same breed, came to the White House shortly after the Obamas first arrived at the White House in 2009. The coats of Portuguese Water Dogs are hypoallergenic, meaning Sunny and Bo won't affect first daughter Malia's canine allergies.
In a post on the White House's website, Sunny is described as "full of energy and very affectionate," whose name was chosen to "fit her cheerful personality."
She was born in Michigan, and is just over a year old, the White House said.
Sunny's coat is completely black -- unlike Bo, who has black and white "tuxedo" markings on his chest and feet.
The Obamas made a donation to the Washington Humane Society in honor of their newest pet, the White House said.
Bo first arrived at the White House in April 2009, after months of speculation about which type of dog the Obamas would bring into the executive mansion. Obama made a campaign promise to his daughters that if he was elected president, they would get a new pet.
While the president originally expressed a desire to adopt a shelter dog, Bo eventually won out for his non-allergenic coat. He came from a breeder that was also favored by the late Sen. Ted Kennedy.
The White House wasn't Bo's first home - he didn't fit in well with his first owners and was returned to the breeder. When Kennedy learned the puppy was available, he arranged to offer him to the Obamas.
Bo spent a month with Kennedy's trainer before coming to the White House. His name -- which happens to be the president's initials -- was explained by the first family as homage to the first lady's father, who went by the nickname Diddley.
Since arriving more than four years ago, Bo has appeared countless times with the Obamas, including in White House videos and photos. He donned bunny ears for a spot promoting the annual White House Easter Egg Roll this spring.
In their blog post Monday, the White House predicted a similar regimen for Sunny.
"We suspect Sunny will follow in Bo's footsteps and keep the President company in the Oval Office, go for walks with the First Family after their 6:30 family dinner and even 'jump up on the First Ladys lap from time to time!" the White House wrote.
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