You can claim that you're going to sue a cyclists for hitting a dog, or a dog owner for causing a cyclist to come off, but in most cases the courts aren't likely to take a great deal of notice unless someone was grossly and clearly negligent.
Most dog owners try and control their dogs, although some are a bit gormless about it. Similarly most cyclists don't speed along cycle paths and expect to never meet a cyclist, small child, or for that matter fox or squirrel, but some still do things that on reflection aren't that smart.
We all have to use the most appropriate behaviour for the environment we're in. Occasionally, whatever we do, someone else may be blatantly and occasionally maliciously dangerous, and that's going to happen sometimes, regardless of whether that person is a car driver, dog owner, or even a cyclist. That's human nature, there are always a few bastards around, and there's little we can do about it.
There are also going to be a few cases, where regardless of what we, or the other parties involved do, an unfortunate set of circumstances are going to occur which cause a collision in which someone (or an animal, assuming you don't consider them to be someone) is going to be injured and/or property damaged. Short of taking an excessively careful approach to everything we do, there's little that can be done about this either.
Some dog owners don't use leads at all times, because 99% of the time their dogs will behave very well, and respond to commands. Some cyclists don't use strictly road legal bike lights and reflectors, because they can be difficult to obtain, and often whilst strictly legal, are actually demonstrably inferior to legal devices. For most of these cases, the authorities aren't going to care. Occasionally insurers may get involved, and they'll try to wiggle out as much as they can. That's when it's useful to get a efficient and well prepared lawyer. Whilst a bike light may not be legal, that doesn't mean that you aren't as, or more visible than a legal light would be. The parties involved in the dispute will both have to demonstrate that their side of the case is the superior one. Merely not having legal bike lights (or reflectors) is not going to be sufficient.
Of course, from a legal point of view, not having legal lights or reflectors is an offence, and there's likely to be little question about that from either side, but then again, in most cases the legal authorities aren't likely to care, unless you're being a complete twunk and have no lights or reflectors whatsoever (and in which case, mostly you're the ones who is going to come off worse anyway).
Note: It's pretty difficult to find legal rear lights in the UK these days. After some discussion in another thread, I think Reelights probably are, but that's the only ones I'm aware of that are easily available at the moment (and which are clearly legal, not just probably because they have another countries standards marking on them). Even the Reelights only become legal because of some pretty sub-standard legislation. The rules concerning lights which only flash, are brighter than four candela and flash at between 60 and 240 times a minute aren't exactly what I consider to be a full and complete spec for a bike light. I'd actually be quite interested to know of any BS approved rear lights, which can be relatively easily purchased currently.