BOZ said:
not so sure that these have any demonic or abyssal connection...
From the Kara-Tur Appendix: "The krakentua is a powerful demon spirit with an insatiable appetite for destruction..."
From the Forgotten Realms appendix II: "Mara are chaotic evil spirits that inhabit great bodies of stone... Mara are subserviant to tanar'ri and other powerful lower planar creatures of like alignment. Further, they can sense the presence of such creatures within a day's ride and move to aid or join them if possible. The creatures cannot gate mara in, however. If more than one chaotic evil planar creature is present, the mara obeys the most po werful. Typical simple commands that mara might receive from their master include orders to search here and there, to find and slay, or hold, a certain creature, and include a mental picture of the quarry or places to search for it.
"The exact mechanism by which mara occupy their stony bodies, and the way in which their spirits make their way from the lower planes to the Prime Material is unknown."
From Planes of Chaos, the Book of Chaos, page 19:
"The Abyss is large enough that creatures besides the tanar'ri can eke out a squalid, brutish life, but a creature's got to be tough and quick to survive. Some are mountains, behemoths, juggernauts of power that even the tanar'ri respect - creatures like bebeliths, bodaks, fireshadows, shinmus, water lords, and the greater varrangoin...
"Others are scavengers that hide in the cracks, scuttling into view only to snatch food and desperately trying to avoid attention. Abyssal scavengers include small fliers like galltrits, gremlins, mephits, shadow fiends, vargouilles, and lesser varrangoin.
"There's servant creatures that make themselves useful to far more powerful protectors: the spying eyewings, the lumbering mara, and trickster quasits. Fetch and yeth hounds also fall into this category."
Of the three, mara and gremlins are explicitly Abyssal natives. Krakentua, although they're chaotic evil demonic spirits, may arguably not be extraplanar depending on your campaign's cosmology.