An Octodangle is only shown once and not discussed further in The Incomplete Book of Dragons
Based on the only image of the Octodangle, it does not look very dragon-like at all. It appears somewhat like an octopus dangling by a thread.
Based on appearance it might be similar to an Electricsticky.
An Octodangle is shown in The Incomplete Book of Dragons in the Cave Dragons section.
It was never mentioned in the Book series. No further information is given.