If it's on private land, then the landowner should be able to resolve it... so if there's a landlord (Council/Housing Association etc) then a complaint that way would work but you need to emphasise the urgency of the matter.
You could talk to the neighbour directly but given that you've indicated that it's not likely to be a reasonable discussion, I'd report it to the police and explain that previous harassment and intimidation has left you unable to talk to the neighbour yourself due to concerns over repercussions.
If the police have a word with him/her it can perhaps be sorted out temporarily until you can get the permanent solution in place. I'm not sure if it could be classed as "disturbing the peace" (it could if you talk to the neighbour and they kick off) but the Animal Health and Welfare (Scotland) Act 2006 places a duty of care on pet owners and others responsible for animals to ensure that the welfare needs of their animals are met.
The Act:
1) makes it an offence to fail to take reasonable steps to ensure the welfare of a protected animal
2) makes it an offence to cause a protected animal unnecessary suffering
3) prohibits using animals as prizes
4) raises the age limit for buying animals
5) increases the penalties for animal abuse and
6) allows the courts to disqualify a person from owning, keeping or being in charge of animals.
https://www2.gov.scot/Publications/2006/10/13113744/1 I would say that point 2 is your way in if needed on this...
I hope this helps and the situation is resolved quickly. When I was a tenant some years ago I had a situation where a neighbour took exception to me parking in "their" space in the communal car park and blocked me in - when I asked them to move they became abusive and threatened violence so I phoned the police. As it was private ground they didn't want to get involved but I had the tenancy agreement ready to prove my case. The policeman asked the bloke to shift his car and he refused. I was driving a rough old pick up truck at the time and I made it clear that either the bloke shifted his car or I would, by sticking mine in 4WD low box and pushing (his was a shiny subaru impreza with fancy paintwork). The policeman confirmed that he couldn't arrest me if I did that because it happened on private land and provided I left my details on the windscreen of the vehicle I wouldn't have committed an offence... I got into my truck and the bloke bolted for his car keys. The policeman was very nice and told me to call again if I needed help. I was a slight lone female in my early 20s at the time and the bloke concerned was late 30s early 40s and built like the side of a house. I made a complaint to the housing association about the intimidation and behaviour of the neighbour so that it was on record. I later found out that 3 other neighbours were afraid to park in the car park at all after altercations with him. He never blocked me in again though.