The word someone is used for human beings but I am having a hard time understanding if it is also used for animals!
If you hear the word someone it almost certainly refers to a human being. Some people will use it fancifully to refer to a pet or other animal, but even then it will be ascribing human characteristics or personality traits to the animal.
For example, if the pet dog comes begging for food, the owner might say, "Well, looks like someone is hungry."
But if out of the blue you said, "Someone came to the door today," nobody would think you were talking about an animal.
Agreed with Robusto.
It could be used for (non-human) animals IF the user is implying that the animals have a "being" that is equivalent to humans. For example, in fantasy literature or New Age Spirituality.
"He had the feeling that someone was watching him." could mean a wolf in the forest, if the writer is ascribing some level of sentience to the wolf, for example.
However, such uses are very specific and outside the norm.
Re: the forest; If a human was the 'watcher' then the writer would use "someone" and this would communicate some form of danger or mystery.
If the writer wanted to suggest an animal, he/she would use "it" which leaves open the possibility of a threat (e.g. a predator) or mere curiosity (e.g. a rabbit or bird).
I would very much concur that "someone" would definitely be applicably in almost all situations in reference to a pet, even if that pet is diverse (antelope/lion/etc). However this usually only applies to people who have had pets themselves, to someone who has never enjoyed an animal's company, nor seen it's desire to understand and better yet actually observe it's learning. To these people the idea of "someone" as in application of any being less than humanity seems to be the act of hysteria.
Having said all that, does anyone know of a word that would apply to both evenly? In any context, would you be able to say "I meet many people" and clearly mean both dogs and humans? I don't think so. You couldn't say "I meet many someones" that sounds terrible whichever side of this debate you are on.
I thought "personalities" maybe, but this sounds misleading at best.
I have to take offense by some of the answers here. "Someone" could definitely be used to refer to a non-human animal, and why not? They're sentient, aware, intelligent, emotional, social, empathetic, and every individual is unique... By referring to animals as things, we degrade them to mere objects that serve humans either as food or entertainment (such as a "pet"). This is an example of speciesism. It's unfortunate that most humans are conditioned to be speciesists from birth, and our objectification of animals is but one consequence of this, to say nothing of the exploitation and abuses (physical and otherwise) we inflict upon them on a daily basis all around the world. Hopefully this will change in the near future, as no one deserves to suffer injustice.