One of our horses is so easy going (and trusting) you can walk between her legs if reaching for a rug strap without her being at all fazed, but the big one is totally different and I still won't walk behind her after 4 years of ownership for fear of her doing this.
That said, even the placid one will give another both barrels in the field if they get too close.
He was lucky to get up from that IMO