A colleague of mine is working on a scholarly paper tracing the history of
"skirted hobby horses" (i.e. a sort of horse costume consisting of a
hoop-skirt-like apparatus that makes it appear that the wearer is
riding a horse) in European festivals and ritual occasions, and
attempting to place their earliest documented appearance.  Skirted
hobby horses are often seen at Mayday and Morris dancing events. I
found an article titled "Masks: Ritual Masks in
European Cultures" by Romulus Vulcanescu in Mircea Eliade's monumental
"Encyclopedia of Religion" (Volume 9, pg.272- Macmillan 1987). 
Vulcanescu, who died in 1999, mentions various horse costume
traditions: the Polish "Koni", the German "Pferdritter", the
French/Belgian/Swiss "Cheval-jupon", and the Romanian "Caiutii".  He
then mentions, in passing, that men in "skirt horses [appear] on an
early Corinthian vase featuring two centaurs squaring off before a
fight". He does not provide a
reference. I need to see an image of this vase, preferably in
electronic form, but a solid library book reference would be OK- I
have access to the library services of a very large research
university.  I have already consulted with "Dr. John" Filpus, who
maintains a website of centaur images on Greek vases- he is not
familiar with the reference.  Also, I should mention that simple
Google searches for "skirted hobby horses" bring up lots of adult
material- beware.