This is a difficult question to answer.
Gauntlet101010 wrote:I think it was the Cyber Fembots (Arcees) by Rabid Squirrel.
No those where actually based on two different 3rd party Arcees from Japan, one was done by JD Productions, basically everything Rabid Squirrel produced where direct KOs or modified versions of other people's items, mostly JD Productions.
Now the oldest 3rd Party figure I know of is a US cartoon accurate Cerebros head (Spike) for Fortmax.
The above mentioned Rabid Squirrel guy bought one from a Japanese collector over ebay, all the seller knew was that it was sculpted and cast by a guy in Japan in the late '80s or early '90s, the original artist is unknown, it is also unknown how many where originally produced.
And since he couldn't track down the original artist, Rabid Squirrel made a mold from it and then sold (badly) resin cast kits from it, just ask robofreak how crappy the casting is on them, he waited about a week or two resculpting it for someone just so it could be assembled and painted.
There might be even older items out there but as of yet I haven't found any.