We have a dog at Wat Kow Tahm who is very disinterested in food. Never seen a dog like this before. You could put it down before the dog and it would eat just what it needs and then goes off. You could try to tempt it with anything but it would eat just enough to keep it alive and nothing more. I don't know but if it keeps on doing that sort of thing, perhaps it will get reborn as a monk. It certainly develops renunciation of food, which is a very hard thing for a lot of humans to do.
I've seen other living beings who have a very loving nature and I've seen some animals who are not so loving. Some animals do very good things for human beings, especially dogs. They save people, they have a detachment, they make a lot of good Kamma. So it may be possible for animals to develop a lot of good Kamma and to get reborn as human once the Kamma that got them to get born as an animal is exhausted.
There is a story in the scriptures, a little story about a frog. The frog happened to be in the area where the Buddha was giving a discourse and there were lots of monks around. The frog became mesmerized by the Buddha's voice and was completely mesmerized listening to what it thought was a beautiful sound. Then a monk came around and accidentally squashed the frog with a cane. Apparently because of that action, being mesmerized by the Buddha's voice, this frog was reborn as a Deva. A Deva is an angel in a higher realm. So whether it's true I don't know.