So there are essentially three uses for the name Aganippe in various green legends apparently, earliest is the Night-Mare, an aspect of Demeter, then the Nymph, and the Spring created by Pegasus. All seem to have some connection to the winged horse legends. Albeit the Night-Mare and the Nymph legends may be more closely related, as in one developed into the other in some form. Apparently that Aganippe means something along the lines of the "Mare that kills mercifully".
It seems that its probably the early Night-Mare that Roberta probably was inspired by, rather than the saxon succubi style one.
The mara/nightmare/mare/succubus stories have there modern equivalents as well...
Some of the alien abduction stories. Ever notice how many relate having some alien sitting on their chest, and not being able to "move"?
I've read that the feeling actually comes from som recognized medical condition, forget what its called, but basically its a deep REM hallucination of sorts. Basically mentally awake during the REM cycle, as in eyes open, somewhat aware, but the rest of the body in deep sleep.
Apparently there are also early myths for a winged horse named Arion that predates Pegasus as well, and very well may have evolved into the later Pegasus legends. Thus both may essentially be the same being.