This makes me think of something my mum told me when I was younger, she was once out in the field where she kept her ponies/family horses digging up rawort and just generally cleaning the field with her parents. Her pony was stood there, looked calm enough so she ran up and jumped on her back ! Her poor pony was so suprised she bucked her off straight away, my mum went flying and was knocked out for quite a while apparently..