Geldings . My mare really doesn't like any of the boys on our yard with the exception of her stablemate Bryn. Any other its ears pinned back and tail swishing. She loves the mares to bits.
Anything under 12hh and coloured - the smaller the worse.
Not sure it's a familiarity thing because he has lived with one or two at a time up until a couple of years ago. It's like he just can't work them out when presented with them unexpectedly