Spectral hounds actually come from the "basic" D&D game; they
were creatures from the Dimensional Vortex beyond the Sixth Dimension
that the Old Ones set up to prevent Immortals from venturing beyond,
but were sometimes found in the Astral Plane on obscure missions for
the Old Ones. They appeared without that background in the Mystara Monstrous Compendium Appendix as astral predators; it mentioned that Githyanki sometimes train them. From there they ended up in A Guide to the Astral Plane and Dungeon #100.
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