Termin wywodzi się od owiec, ale obecnie używany jest bardzo szeroko i przenośnie.

A bellwether is one that leads or indicates trends; a trendsetter. This is an inversion of the original meaning as the bellwether, because of his emasculated condition, always stuck close to, usually following, the flock; his bell alerted the shepherd to its location. So originally the term referred to a reliable follower.
https://en.wikipedia.org/wiki/Bellwether

(...)
a leader, esp. of a sheeplike crowd
https://www.collinsdictionary.com/dictionary/english/bellwet...

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Note added at 3 hrs (2017-09-10 19:56:03 GMT)
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można dołożyć trzeci termin - "head mare"

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The harem group has a clear hierarchy in which the stallion and head mare are the two most central individuals in a very literal sense. The head mare is often referred to as the lead mare, but this can easily lead to some confusion about her actual role. An easy way of thinking about the head mare is as the ‘general’ of the group. Her place is at the centre of the group, where she is protected by a screen of lower status mares. She is the single most important member, and the group’s primary decision maker. Head mares are ‘elected’ to their position by group consensus. The position cannot be captured by aggressive behavior, nor can it be maintained by tyranny. So while the head mare is certainly the highest status female in the group, and is in many ways far more important to their survival than is the stallion, there is no need for her to physically dominate other mares and it would be a betrayal of her freely accorded status were she to do so. Lower status mares may improve their standing by achieving an alliance with the head mare – a win-win element of equine female social politics. - https://www.equine-behavior.com/Dominance_and_the_horse1.htm