the yappy dog head is made out of the same mold as the cow a head.
there are at least seven color variations of mr. ugly.
some older stems have the pez lettering without shadow effect.
some stem colors are unique to only one head.

mickey's nose and popeye's pipe are interchangeable.
there is a variation of the raven dispenser with a 1" nose.
the non-footed pif dispenser was a premium for the french pif magazine and the footed pif for the german yps magazine.
about 3 to 6 years after buying a parrot you will have problems because they want to brood.

for more information about the hobby of collecting pez dispensers check out the pez faq.