The actual origins of these expressions are unclear. Here are two of the most frequent explanations given:
The terms "bear" and "bull" are thought to derive from the way in which each animal attacks its opponents. That is, a bull will thrust its horns up into the air, while a bear will swipe down. These actions were then related metaphorically to the movement of a market: if the trend was up, it was considered a bull market; if the trend was down, it was a bear market. Historically, the middlemen in the sale of bearskins would sell skins they had yet to receive. As such, they would speculate on the future purchase price of these skins from the trappers, hoping they would drop. The trappers would profit from a spread - the difference between the cost price and the selling price. These middlemen became known as "bears", short for bearskin jobbers, and the term stuck for describing a downturn in the market. Conversely, because bears and bulls were widely considered to be opposites due to the once-popular blood sport of bull-and-bear fights, the term bull stands as the opposite of bears.
英语中有句谚语Don't sell the bearskin before the bear was caught（不要在抓住熊之前就把熊皮卖掉）, 这正和人们在“熊市”的卖空头的做法有异曲同工之处。此外，从熊和公牛的习性分析，公牛更有蛮劲，更有攻击性；虽然熊也十分强壮，但和见人就顶的公牛比起来，它们善于沉思，并且易于撤退。
