Right now it is 9th, which is the minimum level you need to get E.B. I don't believe anyone else has a minimum higher.
I see 12 ranks in Ride as a prerequisite, so wouldn't that mean 10th level minimum since the earliest you can have 12 ranks in anything is 9th, or can you meet the prerequisite right as you're gaining the first level in a prestige class? If that's the case, then 9th it is, but what if we want to build our characters up in an unconventional way such as multi-class, or would you prefer the quickest route to the prestige class? In this instance, Fighter.