I think Kishimoto himself might have forgotten about that.

At the time I took it to mean that Shukaku was the spirit of the Sand Priest, sealed and then attached to Gaara. It's unclear whether or not Kishimoto intended Shukaku to be a tailed beast at that point.

Quote Originally Posted by Uchiha Barles
Last but not least, established rules have been BLATANTLY disregarded in this manga and the behavior is increasing in frequency. I have no faith whatsoever in unestablished rules.
Sad but true.