At my barn, quite a few people give their horses a day or two off after vaccines because they know their horse(s) will get sore/stiff/swollen in the area.
Though this year, a couple mares had a weird reaction to what we narrowed to what must have been the rabies vaccine. They laid down and wouldn't get up for a while and had to be forced up to be brought inside. One livened up quite a bit, but another one kept lying down then getting up in her stall. While standing, she'd look very lethargic, stand against the back wall and grind her teeth. BO didn't know what she was doing, but a friend's gelding grinds his teeth when he's upset about something, so I recognized the sound. I've never heard this mare do something like that before and she was completely back to normal within 24 hours of the shots. It was weird.