Cataphracts development

[hoplite14gr]

Сataphract means literally "totally enclosed" - completely covered.
Various bronze age depictions show attempts to increase horse protection by the Mittani and the Hittites.
You are right that the first goal was missile protection.
But as the horse breeding improved and the horses became larger new fighting concepts could be developed.
Though grave findings lead us to asume that certain Skythian nobles had increased armor protection for horse and rider it seems that it was a slow process.
Xenophon talkes and describes Cyrus "omotrapezoi" (who eat with him (Cyrus)) and there is areconstruction in the book "warfare in the classical world". They are reputed to be the first cataphracts.
Bactrian catafracts are mentioned at Gaugamela against Alexander but I am not sure if it is Arrian or Plutarch that talks about them.
Really heavy charging cataphracts are the Seleukidic AGEMA.
It seems that Seleukos combined Bactrian and Macedonian concepts.
The Armenians, Parthians and the Sassanides seem to have been influenced by that model.
In the site www.atarn.org there is a page about a skythian saddle that offers great stability and it is possible that allowed further development.
I think the direct link is in the topic "Greek Cavalry".
Mongol heavy cavalry charged at river Kalka and many belived that charge was their main role (Oshprey campaign "river Kalka")
Hope I helped.
Stefanos