Hi Monty,
Forgive my terminology but...
If "Fishtailing" is to do with the inherrant stability of helicopter flight, then I know this as either:
Dynamic Instability, (where a disturbance will increase in oscillations if not otherwise corrected) or
Neutral Stability, (where a disturbance remains at the disturbed level of oscillation unless otherwise corrected) or
Dynamic Stability (where a disturbance will naturally reduce oscillations to a neutral level).
Engine "Hunting" happens when a change in power (Collective) setting requires a change in engine settings (to adjust engine revs to suit).
This normally results in a temporary engine "Droop" and a follow-up compensatory "Underswing" to automatically correct the engine settings for the required power.
When the Underswing is too coarse this can result in an overcompensation of fuel which can be followed by an undercompensation of fuel, that can then be repeated, going under and over the required setting, resulting in a hunting for the required fuel setting and a "pulsing" of the engine as it seeks the elusive 'middle'.
Hope this is simple enough, and not type specific.