It seems likely Casper is Johann Kaspar Eller. According to J.G. Eller of the "Eller Family Association" in an address to the EFA Family Conference in Portland, Oregon, 1993, there is "...no evidence that he [Casper] came to America." This is also the view of EFA historian Ruth E. Goodman (112 N. Brookwood Ave., Rockingham, NC 28379). Both show Casper as Melker and Jacob Eller Senior's father.
There is considerable belief that Christian Eller (RIN #00767) is a third son of Casper Eller (and that Hook's speculation of Michael Eller as the father of Christian is incorrect). There is a Kaspar Eller marriage on 09 May 1751 (perhaps a second marriage?) to Elizabeth HAGEBOELLINGS at Evangelish, Remlingrade, Rheinland, Preussen. The location doesn't appear to match what little we know of "our" Casper.