At first, the two eyed each other with suspicion. But it wasn't long before it seemed as though Cleo slowly replaced Mzee in Owen's heart. And, occupied with a new tortoise companion of his own, Mzee didn't seem to mind at all.
"They became inseparable like Owen used to be with Mzee, and now they are together, living together happily ever after," said Baer.
Christian was just a 30-pound cub when Ace Bourke and John Rendall found him in a cage at Harrod's department store in London.
But since that moment in 1969, Christian the lion has become a media sensation, capturing the imagination of people all over the world with his story of unlikely love.
Although he made for a peculiar pet, Bourke and Rendall brought him home and cared for him until he grew to a strapping 130 pounds. They kept him in their furniture store and bonded with him as they would a more traditional house pet.
"They just loved each other," said Wildlife Now's Fitzjohn. "Christian adored them. Ace and John basically were surrogate parents to Christian. They fed him and groomed him and made sure he was safe, and didn't get in trouble, and just wanted the best for him in life."
But as Christian grew, the friends knew that their urban accommodations would soon become an insufficient home.
So, with the help of actor Bill Travers, they contacted George Adamson, a conservationist in Kenya, who agreed to reintroduce Christian to his natural habitat.
Bourke and Rendall flew with Christian to the Kora National Reserve in Kenya. When it was clear that Christian would blend with the lions already there, the friends said their goodbyes.
Although the transition was difficult, Christian eventually lived and thrived with a group of lionesses in the wild.
But his first owners missed him terribly and one year later returned to Kenya. They didn't know what to expect or if Christian would remember them. But the lion's greeting said it all.
Immediately, the 300-pound creature raced to the pair and leapt up to embrace them.
"Once they've given you that trust and their affection, they don't forget who you are and what your relationship was any more than a kid forgets who his mother is, however long," Fitzjohn said.
In one of the most extreme examples of odd animal pairings, a lioness in Kenya adopted a baby antelope in 2001.
Conservationists were dumbfounded as to why the lion, a top African predator, would care for its own prey.
"I couldn't believe my ears when I first heard about the adoption. I just thought that's absolute nonsense. Give it a few hours and that lioness will definitely eat the calf," said Saba Douglas-Hamilton, a conservationist with Nairobi, Kenya-based Save the Elephants.
The lioness was dubbed Kamuniak, which means "blessed one" in the local Samburu language. She wouldn't let the young oryx out of her sight.
"It was very moving to watch," said Douglas-Hamilton. But the situation was deadly for both. Caught at opposite ends of the food chain, neither would eat while they were together. The lion wouldn't hunt and the young calf had no way to get milk.
Douglas-Hamilton said she and others thought the lion had experienced some sort of trauma that "clicked a switch" in her brain, and caused her to see "baby" instead of "food" when she looked at the oryx.