animals
I agree with HConn on this one. I don't write books for animal lovers or for animal haters. I do stick to grammar, unless it adds to the story to break grammar, but more than anything else, you have to be true to character.
If the POV character would say "it," you use it. If the POV character hates dogs then you have him call one a mangy mutt and kick it across the yard, then run it down and kick it again for good measure. You might even have him toss a nice piece of poisoned hambuger to a neighbor's dog that won't stop yapping. Then have him chuckle when it goes into convulsions, fluff his pillow, and get some needed sleep.
When he gets up next morning, rested and refreshed, he's in a happy, whistling mood when he clocks in at his job with The Humane Society.
You have to be true to the character and the story. When you start censoring yourself because of what anyone might think or say, you're no longer being honest to the story, and that's always bad.
But animal lovers or not, I'll always use "who" for people and "that" for animals.