I like to imagine that the first two seasons and the first episode of season 3 (when Anakin was officially knighted) of Tartakovsky's Clone Wars are still "canon" since I can't think of anything in them that contradicts the current continuity too much*, and provide introductions to Ventress and Grievous.
* Ventress' origin might not entirely line up anymore, but it's been years since I watched the cartoon so I can't remember for sure.
Season 7 of the CG series also references Shaak-Ti trying to protect Palpatine, which was shown in the 2D series.
There's also a reference in the series to Windu being on Dantooine and taking heavy loses.
One of the new canon novels, (I think Tarkin?) makes a reference to a crashed Acclimator on a planet I don't remember the name of, an episode of the 2D series had a battle there, with a crashed Acclimator where Grevious slaughtered everyone, including poor Shaggy.
Last edited: