Aliens have been testing how their DNA and Earth creatures DNA could be fused. The aliens behind these studies are none other than the reptilians
The reptilians used to inhabit the earth until they discovered space travel this was around the time when humans started recording strange documents about half human half animal creatures all around the world. The reptilians can be found in almost every culture, in Aztec there are half snake half human gods. We have reasons to believe that these aliens are behind this religion and many others. We also know that the gods in said religions are very powerful
Bigfoot is believed to be a humanoid test subject that the reptilians created. When they left earth they stumbled across something more powerful than them and fled to the back to the milky way where to this day are trying to save their species by combining their DNA with ours, and the stronger being is still searching the universe for them to get revenge.