As the chain gets longer, or as it gets more unsaturated, it is more likely to go rancid. Once they cooled, they turned relatively firm, and could sit out for months. They were solid at room temperature, and didn't go rancid because they were highly saturated. That's because fish oil is highly polyunsaturated and goes rancid quickly unless a preservative is used. In fact, all fatty acids in the dog's food need to be protected from going rancid.
Rancid fats can make dogs very sick.
Preservatives, such as vitamin E, BHA, BHT and ethoxyquin, work to protect the fatty acids. Lipid is another term for fat. Many of the marine lipids, which are mostly from deep-sea fish, have a structure different from the oils from freshwater fish. They have more omega-3s in their fatty acids. The key is that these fish eat lots of plankton and other plants that produce the omega-3s.