People only animals capable of meanness?

Not necessarily true. There have been many cases actually of animals killing for other reasons than food or self defense. Predators, Canids and primates in particular, are known for this.
A pack of wild dogs will slaughter an entire herd of sheep if given the chance. They kill simply out of excitement. The pleasure and thrill of killing winds them up so tight that they can’t stop until every sheep is dead. They’re not doing it to be evil, it’s just instinct. Many primates are the same way, and even kill out of hatred or dislike as well. Chimpanzees are well known for this. There are even cases of animals we don’t even think of as high intelligence doing things out of revenge, such as the case of the Siberian Tiger that stalked a poacher who injured him and waited outside his cabin until the next morning, and then mauled and ate him.
People don’t give animals credit for just how intelligent and just how much like us they really are. We have the ability to speak and some of us can reason between right and wrong, but that is really the only difference.

I have a lot of experience with horses and cows, they're not fighting animals (with the exception of intact males infighting or protecting)...it seems they certainly can be mean if they want. They're known to wait until your back is turned and just knock you over, bite, stomp, break something, knock shit over. To wait until your back is turned, combined with the lack of interest or instinct for fighting, implies some deeper thought or intent to what they are doing, I think.

I used to have a dog named Blackie who was jealous and cried if I gave attention to another animal or person. She'd attack the offending person or animal in question. She wasn't defending me or hunting them for food.

I have one word for you, CHIMPS!

I feel like some animals at the zoo are annoyed by people watching them everyday like monkeys/gorillas. After while I'm pretty sure they understand wats going on and get annoyed or pissed, maybe even depressed....

Definitely not. I've seen psychopathic dogs.

My cat used to bring dead mice and the odd chipmunk to my backdoor. Some you could see she snacked on but others were whole.

An animal can be mean in a persons eyes but inside the animals head are animal thoughts and intents.

You've never met a chihuahua! Those little things have more hate in them, than a jock, stuck in the middle of a world of warcraft convention.