Early development of clicker training was done by trainers working with large marine mammals. You can’t physically manipulate a killer whale into position, nor can you give them a leash correction, so a new method had to be developed!
Virtually any animal can be clicker trained; horses, cats, and chickens are some o f the more common subjects.
Recently I came across this post by Mary, the host of the Clicker Carnival, at her blog Stale Cheerios, in which she describes clicker training her goldfish! Watch this video she put up showing her progress.
If a goldfish can be clicker trained, your dog can be clicker trained!