Knowing the previous owner of my SRT was a dog owner, I was surprised that the hair wasn't really a problem (couple good vacs and it was gone) - but the smell - wow! - just seemed to get worse the more we drove it.
It wasn't so obvious when I bought it because the owner had kept his furniture restoration kit in the back (alongside the dog) but the odour of wax polish & turps gradually gave way to that of stale damp dog - and it got stronger every time we used it. All the pet smell treatments we tried didn't help much - then, come service time, the cabin filters were changed and hey presto - fresh air!
So, if your car's a bit stinky - first check when the filters were last changed...