I love this question on Quora because I love animals. I find it uplifting, so let's have a nice moment. Enjoy...
1) Mzee is a Aldabra giant tortoise (Aldabrachelys gigantea), a species which habitat is in the Seychelles. Owen is exactly what you could expect from a hipoppotamus.
Let's start with one of the most surprising pals I had never expected. Because one of them is a reptil, and the other a mammal. You may think that in this case the bond could only flow in one direction (from mammal to reptil), very probably, but it was fundamental for the well being of the young mammal.
They live in the Haller Park in Kenya. In December 2004, tthe terrible tusnami separated Owen, then a juvenile hippo, from his herd. he was rescued and carried to the Haller Park Rescue Center. Having no other hippos to interact with, Owen immediately attempted to bond with Mzee, whose large domed shell and brown color resembled an adult hippo. They formed an odd friendship that continued until 2006, when it was determined that Owen had grown too large to safely interact with Mzee. Today they live in separated enclosures, and Owen is doing a great job in socializing with other hippos, which reveals that the effect of the perceived bonding with the tortoise was very positive for the young mammal.
2) Whales interacting with dolphins: Sperm Whales 'adopt' a deformed dolphin, and whales giving dolphins a lift.
These kind of bondings can occur in the wild. Despite they can be temporal, we can't deny that they defy our understanding of humans as the only ones that can understand another species's animal suffering and help them, and also...what puzzles me more...that they enjoy playing with another species.
Let's start by an amazing example, this video depicts a very rare interaction between sperm whales and an adult bottlenose dolphin with a spinal malformation (i.e. scoliosis). This represents the first time this type of non-agonistic (friendly) interaction has been recorded for sperm whales. We published a description of these interactions in the scientific journal "Aquatic Mammals".
Surprising, isn't it?. But what if I tell you that scientists have filmed whales playing with dolphins...playing, not harrasing or bullying.
Whales give dolphins a lift.
Many species interact in the wild, most often as predator and prey. But recent encounters between humpback whales and bottlenose dolphins reveal a playful side to interspecies interaction. In two different locations in Hawaii, scientists watched as dolphins "rode" the heads of whales: the whales lifted the dolphins up and out of the water, and then the dolphins slid back down. The two species seemed to cooperate in the activity, and neither displayed signs of aggression or distress.
3) Adopted pets can form bonds incredible strong, because they know what is not to have them. When there is a disability included in the equation, the bondings are truly impresionant.
I've talked about wild animals in a rescue center, and wild animals in the wild, but the greatest source of these unthinkable bondings are just average people homes
Pwditat the rescued straycat and Terfel the blind dog:
And, on the other hand, Murdock the blind adopted cat and the family dog who adopted him as his best friend (the name of the dog wasn't mentioned in the article).
Here are the links to know more about them.
4) There is an entire channel of NatGeo dedicated to tell this kind of stories.
Here is the story of a ten y.o hen and a very handicapped toy dog, that are best buddies: