"In his house at R'lyeh dead Cthulhu waits dreaming."
But seriously, an invasion from Venus a billion years ago would leave no evidence we could reconstruct today. After they went extinct, less than a million years later, their microbes would remain. And eventually become us.
That's the most interesting possibility, I think. Probably not just "become us", though, because there's evidence for eukaryotic life, well before then. But it's only fuzzy structural and geochemical evidence. So they and/or their microbes could have merged.
However, the Earth wasn't so hospitable, ~1000 Ma BP. There wasn't much free oxygen then. Not until 600-800 Ma BP, I gather. So if they were oxygen breathers, they would have been living in habitats. But maybe they made the move ~600 Ma BP. That would have been ~endgame on Venus.
I don't see how any of this is testable, however. Some have argued that Octopodidae aren't originally from Earth. But it's iffy.[0] Still, who knows? Maybe they did come from Venus ;)
It's not as if they would still be around ... unless they live at the sea bottom; then there is no reason we would know.
"Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn."
"In his house at R'lyeh dead Cthulhu waits dreaming."
But seriously, an invasion from Venus a billion years ago would leave no evidence we could reconstruct today. After they went extinct, less than a million years later, their microbes would remain. And eventually become us.