Properly speaking the term eugenics applies exclusively to the selective breeding of humans to maximize and minimize behavioral characteristics (it turns out absolutely falsely) associated (by the ignorant) with the (it turns out nonsensical) notion of “race”.
The belief was that behavioral stereotypes (like, for example, frugality, trustworthiness, and work ethic) were inextricably linked to physical phenotypes (like skin color and hair texture) that, via some hand wavy magic around nationality and cultural origin, determined your race.
Eugenics isn’t the same as (or even a straightforward extension of) selective breeding in animals / plants, since no other known organism has ever organized around the entirely cultural artifacts of borders and tribes, nations and races.