During the beta-era of EVE, the 4 races had entirely different titans than they do now (they even served a different purpose - they were more akin to giant mobile stations/clone vats/support facilities than heavy weapons platforms). In lore terms, these are the first-generation "Iapetan" titans mentioned in the wiki's titan article.
Edit: Found some pics.
First off, the Amarr one, the Imud-Hubrau. Originally it was more pointy, reminding me of an oversized Crucifier.
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At some point it mutated into a bulkier-looking Avatar. The first version is the canonical version of it, I believe.
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Next, the Solteur, whom we are all familiar with from the Serpentis event.
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And finally, the Caldari one. No confirmed name on this, although I've seen it referred to as the "Omnya".
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The Minmatar one, as linked above, looks kind of cobbled-together and almost citylike as it was apparently originally based off of the Thukker caravans, which almost matched the original titans' purpose of "mobile support base".
Unfortunately, the site with the best pictures - eve.klaki.net - appears to have since closed, and the pictures are lost. Without those, I've had to use official art for the Omnya and Imud-Hubrau, which really show us very little of them.