Fashion trend as in a feature of clothing that is at least partially ornamental, and widely emulated and adopted quickly for the purposes of appearing appealing.
As a non-historian with a decidedly eurocentric worldview, I'm inclined to believe it's neck ruffs, like during Elizabethan eras.
Your question is hard to answer:
a feature of clothing that is at least partially ornamental
Technically, this could go far back in history - Sumerian reliefs and statuettes show fringe and dagging, for instance, and there are dyes and embellishments (beaded, embroidered, etc.) in many ancient cultures as well.
and widely emulated and adopted quickly
"Quick" depends on your frame of reference. If you're judging communication of the past by modern standards, then nothing happened quickly in fashion until the twentieth century, while the number of fashion periodicals of the 19th century would have astounded the 17th and 18th. Also, how widely? - within a country, culture, social group?
In Western history, fashion (styles deliberately changing over time; contrast with dress, clothing and related material culture in general, or costume, the dress of a specific cultural, geographic, or time-bound group) is generally considered to be begin in the mid-to-late 14th century. To quote Totora's Survey of Historic Costume textbook,
Although one can see instances in the Early Middle Ages when the affluent appear to be following fashion, by the close of the 15th century it is obvious that periodic changes in the predominant style are taking place and that those who can afford to do so are dressing according to the current mode. Details ... were in "a constant state of change ...".
It's difficult to say what the first "trend" would be, because the above definition of fashion is wholly composed of trends. We treat our trends today as something apart from actual dress, things almost imposed on us from the fashion magazines and manufacturers. It's not a useful term when it comes to analysis, because it implies a concreteness that fashion barely ever has: Elizabethan ruffs, for example, grew out of ruffled shirt collars in earlier decades, so there's no line to draw between when they were and were not worn.
But I don't mean to be a total wet blanket! One of the earliest features in Western history post-adoption of fashion that separate fashionable from less fashionable dress would be dagging - cutting the edges of skirts, sleeves, or hoods into tabs or scallops, often lined with another color so that when the edge fluttered the contrast would be seen. it can be seen on the gown in this manuscript illustration from the Roman de la Rose.