Maybe the thing appeared because the AI tried to recruit the unit?

It seems to be a 'simple' "well, you asked me something I don't know about" error. One that means: this id has never been defined in the apropiate id list, and the mod therefore refers to a non existent id. That might indeed be the result from a corrupt download, as it appears that some data got 'lost'.

I guess you won't encounter much trouble due to the error until you encounter a Roman army that 'supposedly' included a unit of this specific cavalry. Don't quote me on that bit though, it's based on one similair experience of mine with a completely different mod.

I'd still reinstall EB, just to get rid of the report even though it might be a harmless one.