Yeah they really exist You need at least one w one FA from that class and one w one from that class and you can make the others from them a few carnivores species need more Essential fatty acid deficiency was first seen in people getting total intravenous nutrition where for obvious reasons they put in only amino acids sugar vitamins and minerals fat emulsions were quite hard to develop as IV formulas The reason for the problem is that the body can make facts but cannot introduce unsaturation closer to the end than w and you NEED w and w fatty acids to make lipids the w DHA is your most common neural tissue fat  Ill see what I can do to put in some general refs from academic nutrition books There is some quackery out there like pills that contain w because olive oil is cheap but most of the fish oil supplements are worth taking actually unless you eat cold water ocean fish regularly  BTW there is some evidence that that bad reputation of saturated fats are from experiments in which animals were fed only hydrogenated coconut oil for their fat source This destroys all essential fatty acids because it destroys all PUFAs On such a diet even DOGS get severe atherosclerosis and they are species highly resistant to it their HDLs are higher than their LDLs So all this is quite fascinating