I would say she may have been rolling and the chicken was in the wrong place at the wrong time except for the feed bucket. It may very well have been "leave me the hell, alone". Too bad about the chicken but I would rather have a dead chicken (all they are good for is eating) than a compromised mare and foal.
They get under her feet all the time eating the grain that falls............and I dont mind about the chicken. I am just glad that I found it, I think the chicken must have got too close when she was laying down for it to have got crushed like that. Yuk.
I am looking out of the balcony every few mins, looking to see where she is : ) I cant do too much going out as my 2 year old has a nasty cough and keeps waking up crying. Hopefully the medicine will work soon..........