Whales, helping out other species without any indications of their actions benefiting themselves has recently been observed!
Specifically Humpback whales. Specifically against orcas or killer whales.
Back in 2009 a team of researchers were witness to a humpback engage a pod of killer whales which where about to kill and eat a seal they had knocked off the ice it had been hiding from them on. The whale was seen floating upside down on the surface with the seal on it's chest being kept out of the water and away from the killer whales.
Another example is from 2012 when another team watched for six and a half hours while up to 16 humpbacks slashed and trashed at a pod of killer whales who where trying to make a meal of the grey whale calf they had already killed.
In all, over 115 cases of Humpback whales going out of their way and against their other instincts (such as in one example where a large swarm of krill was spotted nearby - their main food source - remained ignored by the humpbacks) has been documented by scientists and civilians alike since 1951.
It is thought that the humpbacks are drawn to the scenes by distress calls of the prey animal as they have been known to travel great distances to get there.
The theories on what the humpbacks get out of it is varied and usually centers around the humpbacks in question having encounters with killer whales when they themselves were calves and that the possibility of it being a humpback calf they are saving is worth the expenditure of energy. But just plain old altruism cannot be ruled out.