Unsaturated fatty acids have one or more double bonds. Polyunsaturated fatty acids have three or more double bonds. The unsaturated and polyunsaturated are generally less likely to raise the cholesterol in humans. Dogs don't have to worry about cholesterol, except when they have certain diseases. The unsaturated are more liquid at room temperature, while the polyunsaturated fats are definitely liquid at room temperature.
Shorter chains tend to be more liquid.
For example, acetic acid is a short fatty acid with only two carbons. It's found in vinegar. Butter has much butyric acid, which is a four-carbon fatty acid. Much longer fatty acids, with 16 and 18 carbons, are found in beef and pork, and are relatively solid. Fish oils are 20 to 24 carbons long.