What exactly is a "chaser" pin?
I guess you could consider them as either bonus or alternate pins, since some chaser pins will be of the alternate configuration of an existing design within the set, as your unpainted pin example illustrates. Others will be of similar design, but may be a completely different character. Some chasers are subtle and there may simply be color variants or a single item that is slightly different from the same looking pin in the set.
Sometimes chasers are refered to as completers, as they often, in the case of hidden mickey pins, are released at the tail end of the distribution year.