At last!, the secret is out. It was reveal that the Secret piece to Vol.6 Masked Collections is Hyper GATACK Form. Almost similar to Hyper Kabuto, Hyper Gatack also have HUGE horn on its head and it made it looks weird/funny in a way.
Hyper Gatack ONLY appear once and only in the Special DVD title Hyper Battle. Getting this piece would be hard as is with previous releases. Ratio assortment for this secret would be 1:64 as compare to 1:32 for Hyper Kabuto.