Begginings and Creation
Soon after Orochimaru left the Akatsuki he had devised a plan to create his own tailed beast army to destroy them.He gathered thousands of different animals and did different types of experiments on them:Fusing them,Imbuing them with Chakra,Giving them elements and even fusing them with people,to limited results though not the ones he wanted.He grew angry and abandoned the project beleiving it to be impossible but one of his scientist asked orochimaru to let him carry on experimentation orochimaru told him to do as he pleased and the scientist,which was named Ugaru Suyamoto continued trying different things.One month later after many failures he managed to combine a whole animal species into one body,implant it with DNA from thousands of ninja (Part of another experiment he was making in which he made Shinjiro and his brother)and give it DNA of the other tailed beast's and their Chakra,Thus making the first Pseudo-Bijuu,The 2 Tailed Panthera.
After Creation and Implanted Into Shinjiro
At first it seemed to be completely silent and unresponsive though after awhile it started moving and speaking frequently.It semmed to grow atatched to another experiment that was being made,he also had DNA of many people.After Shinsettsu escaped he destroyed the lab and tried to kill Shinjiro but the 2 tails protected him.It seems it originaly had 12-tails one for every beast and his own 2 tails,However he gave shinjiro his other 10-tails to survive then entered shinjiro's body and controlling him defeated shinsettsu and drove him away.Afterwards he officially became Shinjiro's Bijuu thus making Shinjiro the 2-tailed jinchuriki.It is revealed that The two tails has regained its other tails inside shnjiro's body however in all its apperances it chooses to apear with 2 tails to symbolize their friendship.
Humanity's dark secret
To make the 12 tailed panthera Ugaru Suyamoto sacrificed 100 humans and nearly a thousand different animals,same thing with shinjiro and his brother.