English, like other languages, has many phrases, idiomatic and otherwise, that refer to animals. Some are neutral, or even positive (eg: "pretty as a peacock"), others, less so. For example, although pigs are intelligent creatures, people often use the word "pig" to refer to someone who is greedy, dirty, or unpleasant. This usage of "pig" creates and/or reinforces the idea that these animals exhibit these negative qualities, which is not true. In fact, this particular phrase is a classic example of misattribution of human qualities (in this case defects) to animals. There are quite a few things we got wrong about pigs in our pig-related phrases.This article about pigs dispels this and other myths, and lets you know why other pig related phrases, such as "to pig out", are also inaccurate. Similarly, when we refer to someone as a "rat", we are saying that person is disloyal, deceitful, and/or just plain horrible. Why use "rat", an animal that has nothing disloyal/deceitful about it, when you can easily use, say, "politician"?
A dog is one of the most noble, faithful animals there is, and yet when we call someone a "dog", we're not referring to that person's nobility and/or faithfulness, but to his wickedness, often of a sexual variety. To "beat someone like a dog" is still often used to refer to giving someone a nasty beating. Do we really need to verbally associate (and thereby reinforce) "beating" with a dog, a mule, or any other animal, when violence towards animals is still a big problem in our society? While thankfully not used as much these days, "there's more than one way to skin a cat" used to be a pretty common phrase referring to there being more than one way to do something. This begs the question: Why would anyone use such a violent image, when so many other less disturbing ones could be used in its place? The answer is quite simple. Back when the phrase was first coined (mid 19th century), the sentience of animals such as cats was not something most people knew or cared about, so making light of such an animal's professedly insignificant life was completely acceptable. We, however, are living in 2015, and in our modern world, violent phrases that make light of animal suffering should not be used. "Shooting fish in a barrel", which refers to obtaining something without any effort, is another such phrase. Although seen as humorous by many, it is pretty unambiguous in its acceptance of violence. The underlying idea here is that it's OK to kill fish, it's just the method that matters. Fish and other marine animals continue to have a hard time getting an empathetic nod from people, so we don't really need to use phrases that reference violence towards them.
Birds too, are not immune to negative metaphors. "Bird-brain" is still used to refer to someone we think is stupid. This ignores the fact that crows, for example, are extremely smart animals. We are learning more and more about the intelligence of birds, and using phrases such as "bird-brain", and the horrible phrase "to kill two birds with one stone", either downplays these animals' intelligence, or reflects an unacceptable matter-of-factness about killing them.
These are only some examples of the way we use language to either misattribute human qualities to animals or to trivialize animal life. Many of these phrases came to be because of the "it's just an animal" approach to other sentient beings. These days, we should know better. We should respect sentient life, and not using words and phrases in ways that perpetuate archaic and erroneous ideas about these animals is a good step along that path.
