Sounds like a caterpillar might have landed in your skimmer.

Old House, take a look at the linked site below.
At a guess I think you may have found a Cutworm.
They are usually terrestrial but it may have fallen off a plant into your pond and ended up in the skimmer.
Cut worms come in a variety of colours,some with stripes, some just plain.
"Horton"

Here is a link that might be useful: Cutworms

Thanks Nancy and Horton - the cutworms look similar, but not quite the same. I agree, now, that it was probably just some land-loving escapee, rather than a fish sucking parasite. I thing, though I am not a biologist, that the parasites are considerably smaller than this guy was.

The thing that did and still does throw me though was the way he moved, he pulsed to propel himself forward, rather than wiggle (Like a worm) or "step" as an inchworm does (where they arch their body, then flatten, to move ahead)

Ill keep looking for info - there are just SO MANY worms out there ...

OH, I think you are correct by the way you describe it's forward action, that it is unlikely it is an aquatic parasite.
I have found some of those worm/caterpillars in my garden from time to time, they arch their backs as they move along but I have never encountered one in the pond.
"Horton"