In about 100 years over a wide area (10,000 sq. miles),
1000s of Indus era seals have been found. None of them
depict any horse, and no horse equipment or remains
have been found from the mature Harappan period.
The Rg Vedic Aryans' and their use of horses is well
recorded, and the Rgveda is dated to somewhere around 1000 BC.
This puts almost a gap of almost 8 centuries from
the ingression of Aryans into India. See prof. Michael
Witzel, Harvard university talking about the date of
the Rgveda's first collection to 1000-900 BCE http://listserv.liv.ac.uk/cgi-bin/wa?A2=ind0011&L=indology&P=R5692