Whoa Nellie

Hi there,
Where did "Whoa Nellie" come from and does it mean anything other than the obvious...i.e. slow down or stop to a horse named Nellie?
Thanks in advance.

I'd say the meaning is straightforward.

I don't know if this is a connection. On the Roy Rogers' show there was a jeep named "Nellie Bell." Does anyone remember that? I googled and found one instance of someone saying "Whoa, Nellie Bell!"