Language changes over time, always has. The "correct" form is just a snapshot of what has gone before, and what some people sat down and decided was canonical based on that. But people change language, it's not static. In recent years, there has been a trend to see animals not just as things, but as living beings worthy of respect and consideration. To people who feel thus, referring to any animal as "it" just doesn't feel right. They are "he" or "she", and I see no problem with "someone" either. This doesn't mean we are anthropomorphizing (i.e. attempting to treat the animal as human), or calling them "our babies" or whatever; we are simply recognizing something vital and fundamental.

I guess there will always be people who see animals as being something less than humans, but I'm just explaining the mindset that objects to animals being referred to as "it" - we are all animals, equally worthy of respect. Of course that view understandably raises all sorts of mental discord if you've been brought up to think that it's ok to eat animals and treat them as things to be used for our convenience and pleasure; if we saw them as anything more, then we wouldn't feel as free to casually abuse them in the ways that we do. So the current status quo kind of makes sense, in that context - but I see it as speciesism, in the same way that looking down on some other race of humans (which was once completely socially accepted - see slavery) is now seen as racism. They are both coming from the same place, really.

Some current books that go into this in much more detail than I can here, for anyone interested:

Wild Justice: The Moral Lives of Animals
The Animal Manifesto: Six Reasons for Expanding Our Compassion Footprint
Animal Wise: How We Know Animals Think and Feel
The Emotional Lives of Animals: A Leading Scientist Explores Animal Joy, Sorrow, and Empathy — and Why They Matter
The Dreaded Comparison: Human and Animal Slavery

Neil