I agree that there are handling and environmental factors, but there is a very big genetic/heritable component to temperament and personality. I have had a small breeding program with many of them being by my own stallion over the years. The similarities are uncanny. None of the horses I have bred by outside stallions share so many of these same characteristics even out of the same mares, and being raised in identical ways. The offspring of my stallion have been smart, trainable, sensitive, usually sensible, and virtually ALL of them are super friendly and personable and are constantly getting into shit! They are always chewing on something, pawing at something, investigating something, destroying something, etc. My other babies are not like this. I like to say it’s because they are intelligent and get bored on 24/7 turnout and just find stuff to investigate. My husband likes to say it’s because they are stupid and have to constantly break things. Lol. The debate rages on. But we have started so many horses and I had four full siblings, and the similarities in his offspring far outweigh the differences. This is not nature since we raise them all the same, and the other babies are completely different, as are the outside client horses we start and train.
There are also well known temperament characteristics passed down from other better known and more prolific stallions (the mare contributes greatly too, but might have have a maximum of 10 offspring, vs potentially 1,000s by the popular stallions, so a much bigger group for comparison). As someone else said- ammy friendly, professional rides, hot, lazy, funny mouth, careful, chicken, brave, spooky, etc. These are not coincidence IMO.
