Managing pigs with other species
Although we would always recommend pigs have pig friends, sometimes pigs social needs can be met by other species as well as, or if neccesary, e.g. because of space limitations or fear of other pigs, instead of other pigs. A couple of species are worth talking about specifically.
First, horses. Generally the advice is that horses and pigs don't mix. Horses are often scared of pigs, the high-pitched squealing doesn't help! If you want to keep pigs near horses the best thing is to have them in separate but adjoining areas so the horses get used to the pigs but some horses will never be happy and we have had people give up their pigs because the neighbours' horses were too terrified to go in their field or were too scared to go past the pigs to their stables. So if you have horses you need to be prepared for the possibility of keeping the pigs in an area well away from them if things don't work out.
The second species is dogs. It is quite common for homes to have pigs and dogs and sometimes it all works out well. Unfortunately this isn't always the case and aggression can arise between the two. In the USA this is actually becoming a major issue for pet pigs. An increasing number of pet and abandoned pigs are severely injured or killed by dogs. Dogs are a predator species and pigs are a prey species, and they don't speak the same language. Clearly it all depends on the dog, some have about as much predator in them as a cuddly teddy bear, but the advice is do not leave pigs and dogs alone together.
Aside from dogs and horses, pigs can and do make friends with chickens, cats, rabbits, etc but again it isn't guaranteed they will always get on so at least at the beginning, introductions need to be supervised. Sometimes just the sheer bulk of a pig might be a danger to a small animal. And pigs do get grumpy and grumpy usually means barging and biting. Pigs have also been known to help themselves to a snack of a passing chicken (pigs are omnivores)! The bottom line is, always do careful introductions, supervise when they are together as far as possible and/or keep in adjoining areas, and have a fallback if it doesn't work out.