Sort of a TAAT, but I didn't want to start quizzing this on another thread.
We had cocker spaniels growing up, and I remember being warned off solid coloured cockers because of cocker rage.
In the last year I spoke to a vet nurse who specialises in training (using clickers and positive rewards, not dominance stuff), who said it was a myth, some breeds of dogs (not just cockers) can tend to be a bit nervous, and if they are trained using dominance methods (very popular throughout 80s and 90s) there is a good chance that the owners won't recognise the subtle signals the dog gives that they're not feeling happy, and the dog may seem to suddenly turn with apparently no reason.
Is it genuinely a thing, or just a misunderstanding?