Around 2002, a 40K scale resin Warp Hunter was produced. It was sculpted by Michael Bard and produced by an amateur fan project calling themselves Deep Strike Miniatures as an "Alien Tank Hunter". Also around 2002, Dragons Den Armoury produced a single 40K scale “Gabriel” Knight Titan that looked remarkably identical to the GW Epic Paladin Knight. These were only sold on ebay at US$250 and were only available for a couple of months until they were closed down for breach of GW intellectual property.
Around 2004, the same group started Dreamforge Designs and created two Titans which stand about 4 times as tall as an Armorcast Warhound. (pictured above - yes that is a mini next to his toe) Two versions were produced using rapid prototyping [very expensive! ] from CADD drawings. Initial price was $750.00 per model. 6 models were sold before they were closed down for breach of GW intellectual property. It seems only one variant of the two was sold and the prototype casting of the second Leviathan model may be the only one in existence.
From 2007, after market heads for the Armorcast Reaver Titan started to appear on ebay from sellers "chrono38" and "westland48".
Currently there are a number of 'legal' Titan variants that are available for use in GW events, and as we have seen in this series of articles - most are legal to play because they were produced under GW license at some point.
There are many different models that appear on ebay and in other venues from time to time, hopefully this series of articles will help you to distinguish them from each other.