Found it. Warning: it's very long (sorry) and very freaky, and if anyone can come up with a rational explanation I'd be extremely grateful because I'm generally a pretty rational non-woo where's-the-evidence person, but I've never been able to explain this and it still makes my heart beat faster and the hair stand up on my arms when I think about it, years later.
I was pet sitting for my friend several years ago. She had moved into a sort of small holding right on the edge of a village in the SW, with a huge garden that backed onto trees. At the time she had a right menagerie - chickens, ducks, a couple of Shetlands, cats and four black retrievers, three fully grown and one a half grown puppy. Originally DH had been going to come with me as a little holiday but the dates she ended up having to go away for work clashed with a couple of his medical appointments so he stayed at home with our dog and I went to petsit alone.
It was about halfway through my stay, a couple of nights to go. Late evening, already dark when I heard a massive commotion from the chicken shed, banging and thumping. I assumed a fox or something had got in so went out to check. As I was halfway across the garden the noise stopped instantly as if it had been shut off; by the time I got to the shed all was calm and the chickens were all settled, mostly asleep. No sign of any intruder or disturbance at all, nothing to explain the noise or any indication that the chickens had made a noise. Bit freaky but I didn't (and still don't) know much about The Way of Chicken so I locked up again and left them to it.
As I went back in the house a small black shape ran past me out of the back door and I realised the puppy must have got out. It streaked across the garden and off towards the woods. Cue much cursing, then calling her name in vain. More cursing when she didn't come back. I grabbed a torch and put one of the other dogs on the lead, partly for protection, partly because I thought the pup was more likely to come back if I had one of her canine companions with me and partly because I didn't fancy my chances of finding my way back to the house on my own even though there were a couple of vague paths that I'd followed when walking the dogs throughout the week.
Off we trudged into the wood along one of these paths, me calling pup's name at intervals and trying not to imagine murderers and rapists behind every tree trunk. We got to a point where it felt like the trees were starting to thin out and I remember thinking that I didn't remember a clearing on this path and we must have gone wrong somewhere when the dog with me slowed right down and started to resist going forward. I tried to jolly her along - while my stomach suddenly dropped like a stone - and she started growling, a really low serious rumbling growl. By this time I was practically shitting myself. I tried shining the torch ahead but the beam just sort of bounced back off the darkness if that makes sense? I got the sense of something - or somethings - moving but just sinuous deeper black shapes against the blackness and always on the periphery of vision. (The hairs on my arms are standing up again just remembering how completely and utterly terrified I was. I have honestly never known a feeling like it.)
At this point the dog sank right down, still growling, hackles up and refused to budge. I muttered something like "Jesus, you have got to be kidding me" and this ugly gurgling inhuman sort of voice hissed, right up close as if someone was right next to me "don't say that name". At the same time there was a horrible snickering sort of laugh. I cant express how utterly petrified I was. I can't remember having any coherent thoughts apart from the word "evil". That's the only clear thing I can remember. Me and the dog were frozen to the spot with pure fear. Then a different voice, really commanding, said "GO. BACK." That sounded more in my head but echoey, where the others had sounded out in the air IYSWIM?
Wherever it came from it did the trick. Me & the dog turned and belted back through the woods. She basically towed me, I just clung onto her lead stumbling to keep up and sobbing with fear. I lost the torch somewhere on that wild run but there was no way I was stopping to find it. How I didn't run blindly into a tree I'll never know, she guided me I guess. I can remember thinking desperately that I mustn't let go of her lead or "they" would get me.
When we got back to the garden she suddenly stopped - I did fall over her this time, onto my hands and knees - turned around and started snarling, proper teeth bared, rabid-looking snarls, back at the trees and the darkness. I thought I heard the snickering again but the blood was pounding in my ears so hard I can't be sure. I scrambled up and ran to the back door and she followed me but backing and snarling all the way as if holding something at bay. Oh, and the chicken shed was banging and thumping again. I got the back door open, me and her belted in, I slammed home every bolt behind us. The other dogs left behind were staring at the door and growling too with their hackles up and when I saw all three of them, puppy included, acting like that I started to cry properly because I honestly thought I was trapped in some horror film nightmare and was going to die. I don't know - I still don't know - what the black shape was that ran past me out of the house and triggered all of this because the puppy was right there in the kitchen.
Anyway I made sure every door and window was locked and bolted, I turned on every light in the house, I wandered round mumbling all sorts of weird half-religious half-spiritual shit to ward off evil spirits. Gradually the dogs settled down and stopped growling, and eventually stopped glancing at the door. Funnily enough I didn't sleep for one second that night and I rang my DH and begged him to come over the last couple of days. I know I didn't dream it because I was covered in scratches from running through the woods and had grazed hands from where I fell over the dog in the garden.
Nothing like that has happened before or since and I hope it never, ever does. It was the single most horrible, terrifying experience of my entire life.