Here is the exact quote from Why is a Red Herring Red? by Mitchell Symons (2020):
Consequently, when the herd is hunted, it is the slowest and weakest ones at the back that are killed first.
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In a comment John Lawler wrote:
The plural in ones comes from the fact that herd is a group noun, and consists of individuals together.
Also in a comment BillJ wrote:
The pronoun “it” is not referential here, but a meaningless dummy element functioning as subject in an it-cleft construction, where the foregrounded element is complement of "is". The pronoun is really just a 'placeholder' for the variable that is defined in the relative clause; it does not have the number properties of the NP headed by plural "ones”.