A herd was a group of animals. Animals which came in herds included elephants (TV: The Daleks' Master Plan, The Hand of Fear) buffalo (TV: Ghost Light) and cattle. (TV: The Impossible Planet) Danny Bartock described the Ood as a "herd race". (TV: The Impossible Planet)
Captain Jean-Luc Picard likened the prospect of the USS Enterprise-D engaging a Cyberman Fleet to a mosquito against a herd of elephants. (COMIC: Assimilation²)