It seems plausible that what 'works' in breeding dogs and horses will work in humans. This overlooks the main problem - no-one is interested in breeding humans with big floppy ears or whatever. The eugenics movement struggled with this problem, and never overcame it.
They hoped to breed people healthy in mind and body, and imbued with civic-mindedness - really. They were then faced with several problems. The 'simple' health attributes are a product of many, many genes, and the environment.But this wasn't the main concern, which was political.
They literally hoped to breed out certain social problems, like unemployment, alcoholism, and a lack of civic duty. I am not making this up, in the early days of genetics, they hoped that these traits were genetically determined and heritable. As a spoiler, they aren't.
But before the eugenicists even got that far, they had to come up with definitions, ones which allowed measurements of individuals, and possible to carry out on large populations. The IQ test was taken up to fulfil this aim, and it is highly problematic.
IQ tests only correlate with some ideas of intelligence, and it isn't clear how much of which ones are heritable. This didn't go very far towards the eugenicists' aims, because what they really wanted was a scientific model which conformed to their own socio-political prejudices.
They wanted more people like themselves - and they were almost always middle-class urbanites - and fewer of the people they feared - the urban poor. The attributes driving these social attitudes have social, not genetic, causes.
Eugenics is a pseudo-science, because it starts with political beliefs, and makes up the science to conform. When research is done, it unsurprisingly produces results which support the political aims which the eugenicists wanted in the first place.
Also, since it is driven by fear of different people, the efforts to define and measure good qualities to select for were always in second place to the desire to have fewer of the undesirables in the population. It is an inherently exterminatory politics.
