all ruminants' meat & milk contains traces of vaccenic acid, which is a naturally occuring fat and an isomer of oleic acid (the predominant fatty acid in olive oil).
http://en.wikipedia.org/wiki/Vaccenic_acid
it is converted by humans into CLA, which is a good thing:
http://en.wikipedia.org/wiki/Conjugated_linoleic_acid
incidentally, the saturated isomer of oleic acid is stearic acid, another 'good fat'...
The 'bad fats' are other fats with trans bonds rather than cis bonds that do not occur anywhere in nature. Humans have been consuming the milk & more importantly, meat, of ruminants since the very first deer or woolly mammoth hunter. The trans fats of concern, resulting from the partial hydrogenation of soybean and other oils, were never consumed by humans in any quantity until the commercialization of Crisco one hundred years ago.
the jury is still out as to whether or not the stearic acid that results from full hydrogenation is as good for you as natural stearic acid or as bad for you as other artificial trans fats.
[edit: Guinness screws with my ability to punctuate properly]