One interesting source of natural trans fats is from ruminants: dairy, beef (grass-fed more so than grain-fed), and the like.
The symbiotic bacteria in a cow's rumen ferment cellulose into fatty acids. Did you know cows are fativores? They may swallow grass, but what they actually absorb is mostly fat.
Some of these fatty acids are trans fatty acids, such as vaccenic acid.
However, since humans have been hunting ruminants for millenia, we have co-evolved with those ruminant trans fats in our diet, to the extent that the evidence seems to be that they are benificial (or, to put it another way, that their absence is detrimental).
So next time you read the label of your feta cheese, and see "Trans fats: 1g" on it, relax. That most likely just refers to natural ruminant fatty acids, which unfortunately need to be totalled up and labelled under the trans fat heading. Iit would be less confusing if regulation excluded ruminant fatty acids, since they are not what we are concerned about.