The major source of dietary TFA is Industrial i.e. partial hydrogenation of vegetable/fish oils
1. Small amounts of trans-fatty acids do occur naturally in animal fat e.g.
Meat of ruminant animals (like cattle, sheep, goats)
Milk of ruminants and dairy products from such milk
These account for a very small percent (2-5%) of animal fat.
Some researchers claim animal origin TFA to be even beneficial.
Moderate intake of animal fat doesn’t seem to be harmful.
2. When the oil used for deep frying is heated over and over, partial hydrogenation of PUFA (Poly Unsaturated Fatty Acids) takes place, resulting in formation of trans-fats
Trans Fatty Acids (Trans fat): http://www.ihatepsm.com/blog/trans-fatty-acids-trans-fat