MyHaven .... have to agree with Teila that you might have double trouble around there - foxes - - - and feral cats, which can grow mighty large.
I doubt a fox would ever enter a house to beat up a cat, with all those human smells around in the house. A large male feral tom cat might though.
I hope anti-biotics will fix all the bacteria that can occur in puncture wounds on a cat ( or anything ). And I trust your lovely cat recovers well. I am sure she will, with the vet. intervention you immediately sought.
Abcesses can form quickly from puncture wounds ( teeth in particular ) .... in cat attacks. And maybe your little girl cat was too overwhelmed to fight back, or did not have the size or 'feral' instinct, to do so.
My only advice ( which doesn't sit well with a lot of people ) ... is - - - lock all your precious animals / birds up at night, with no exception. Manually let them out to toilet or scratch around, in the a.m. but ( if possible ) not too early. Foxes don't mind the dawning of a new day, and will continue a hunt if they are hungry enough - into the daytime. Mentioned a while back, of a friend who lost her Siamese cat to a fox, at midday - which she saw happen. Grief stricken she was, and in shock.
We can't be held to ransom by foxes though, so we can only all do the best we can for our furry and feathered loved ones.
And as you have said again, that you have snakes as well, it's still my bet it was possibly a small snake that bit your chick, injecting its' poison. Rats also dig like crazy - large holes to get into wherever they want ... and they also chew chunks away on wooden doors ( I discovered ) to gain entry to chook food. A large rat might also kill a small chicken, but how through small wire, I cannot imagine.
Cheers .........
Absolutely agree, everyone should be locked up at night to keep them and our wildlife safe.