I don't care what the fucking date is .... Bonfire Night, New Year's Eve, Chinese New Year .... or whatever. Whilst it may be traditional to have fireworks at those times, it doesn't give you carte blanche to set off explosives right next to large animals contained in a field, who have nowhere to escape - unless it's over/through fencing in a blind panic - and who could therefore injure themselves (or worse) trying to get away .... not to mention the possibility of further mayhem should they escape on to roads.
Good god ..... regardless of whether you know the animals' owner, regardless of if you know where they live .... you just don't bloody do it unless you've expressly informed and agreed that it's okay to do so. And if you can't track down the owner then tough, surely the animals' wellbeing (and their owner's livelihood) takes precedence over your "right" to enjoy a few fireworks. It's not as if there aren't any organised displays if you were that desperate.
OP - I really feel for you and hope the damage today isn't too great. This is absolutely all about courtesy, consideration and compassion which your neighbours are obviously completely lacking. Yes - we all know it's Nov 5th blah blah blah but fireworks are on sale for weeks before and after for a start - and while most pet owners know to keep their pets in, it is different for owners of larger animals who aren't so easily transported elsewhere - and who typically tend to be kept in more remote areas where it's far less likely they'd be exposed to fireworks anyway. Am not sure what the OP is supposed to do "just in case" - she'd otherwise be moving them for days/weeks on end. These people were very very irresponsible - and yes, cruel - to do this without any consideration.
(As an aside, I went to a display this weekend. Some total fucking moron had bought his dog with him - to within 6 feet of the barrier round the bonfire and launch area. I didn't realise he had until the 1st rocket went off (or else I'd have confronted him) and I saw this blur whoosh past me. Thankfully, it was on a lead and thankfully, this man left at that point or else I'd have ended up in a row with him and would have taken the dog away myself (or involved the police). He came back shortly without dog, so very likely poor thing was shut in a nearby car where it wouldn't have been protected from hearing all the bangs and would be petrified anyway.)