yes I dont remember where but I know it was said that shukaku is the spirit of an old priest...
You are correct. If you watch the anime, they show the jar in which Shukaku was stored. It is almost if each Jinchurriki gives part of himself to the Shukaku over time... because Shukaku is stored in that 1 jar inbetween transfers between different Jinchurriki's.
I was just watching the anime episodes with the "Hidden Star Village" ... and noticed that large quantities of chakra, depending on the type of chakra, take on specific forms... In the star village case, the purple chakra took on the beast form of a dragon's long neck and head.
Another example of chakra taking a form... is when Shikamaru fought the girl from the hidden sound village, when trying to chase after Sasuke. The girl's flute puppet, had a technique, where "Spirit Energy chakra worms" appeared, and according to Shikamaru, "are in an unbalanced form, because they lack Physcial Chakra to stablize its form" and that is why they were wanting to feast on his chakra.
This is the ying and yang balance of the chakra beasts, including the Tailed Beasts that have aquired this balance and took a physical form.
In summary, depending on the nature of the chakra, it will gather and take some type of form.
Shukaku could have been created by the spirit of a powerful ninja, that died, and was not able to "sleep eternally" and that is why Gara was possesed by a monster that would not let him sleep. It sort of makes sense.