There are several moves in the Psydex that are listed as egg moves for certain Pokemon, but without an explanation of how to inherit them - there's no "possible chain", and apparently no compatible Pokemon can learn the move.
After a bit of digging I discovered that most of these moves can only be inherited by importing a male Pokemon from a previous generation (where there was a TM or move tutor that doesn't exist in later games) and then breeding. In other words, the Psydex doesn't explain them because within the game itself, the move can't be learned by any compatible Pokemon - they can only be learned with the help of Pal Park or the PokeShifter.
I discovered, for example, that Deino can't learn Dark Pulse as an egg move unless the TM from DPP/HGSS is used on a male Gyarados, which must then be imported to Black/White before breeding with a female Deino.
I call these moves "Intergenerational Egg Moves" and I've been doing some research into them, using the TechDex as a reference. Because there are so many, especially from the more versatile TMs that don't exist in Black and White, I think a new category of parents needs to be added to the Psydex Breeding section: "Traded Parents".
...I also discovered an error in the Breeding information - http://www.psypokes.com/lab/breeding.php
- Cryogonal, Klink, Klang, Klinklang, Golett and Golurk are genderless, and should be in the Gender Unknown egg group rather than the Mineral egg group.