She's got a couple of bald areas in the general area of the bump, so I would definitely suspect some kind of injury at some point. It could have been prior to her current owners, which would be why they don't know about it. If she's been shifted around, it could be the reason for it has been lost somewhere down the line.
I would go check her out, thoroughly check out her movement (to rule out any neurological issues), attitude, etc; and definitely get a vet check.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."