Could be the horse sensed something, but since the deer remained relaxed and in the area, doubt it.
When we take our horses to the mountains, they see elk for the very first time
Far as deer, we have lots, so the horses are used to seeing them.
Once, while riding my mare out at dusk, along a wooded fenceline, three big bucks jumped out right in front of her. She just gave a start and spooked in place, then we continued on
Came across a grizzly bear on a ride, and my gelding just stopped and stared. Luckily, that grzz was just as happy to see the last of us, and took off in the opposite direction!
Once, riding down the road towards home, in the fall, there was abig bull moose standing next tot he fence, and being a dumb moose, took interest in my gelding, as the moose was in rut. My horse stopped and looked back at me, as if to ask, ‘now what’?
I knew I could not just try and out run that moose, so called a bluff by un tying my coat from the back of the saddle, waving it in the air and asking my horse to charge that moose. My horse did, because he trusted me 100%, and the moose left
Since the deer were not alarmed, and they are a major prey species, I would have made my horse go by that spot, by keeping forward motion, maybe counter flexing him some until he got by the deer. Once a horse sees he can move deer or cattle, etc, he gains confidence
A lot also depends on how calm you remain, because if you get up tight, fearing the reaction of your horse, the horse then thinks that there really is something to be concerned about, as his leader is also apprehensive
I know, easier said then done, but if you manage to remain both calm and strong, your horse will gain confidence form you
Where I ride, the single track doesn’t allow my horse to see whats coming around the bend, so I always say “people” when people are coming and pat him on the neck to reassure him. For deer, I whistle both to cue him that’s its just deer and to scare the deer off - i will also have him trot into the deer to scare them off. That gives him confidence.
But I don’t ride after dark any more since mountain lions have been sighted and we had a mountain lion kill on the front lawn of the barn itself - just a rib cage and a part of a leg bone - the rest of the carcass stripped clean.
