Golly, gosh! Special horses (3-4)
I believe the answer is:
gee-gees
'horses' is the definition.
(I've seen this before)
'golly gosh special' is the wordplay.
'golly' becomes 'gee' (I've seen this in another clue).
'gosh' becomes 'gee' (I've seen this in other clues).
'special' becomes 'S' (common abbreviation as in Special Air Service).
'gee'+'gee'+'s'='GEE-GEES'
(Other definitions for gee-gees that I've seen before include "Young children's horses?" , "Horses (informal)" .)