Bigfoot(s) are just Alien Pets that Dad 'forgot' about when he flew off in the Family Mobile Home.
Luckily, several visitors have 'accidentally' lost a few pets (is it because they STINK REALLY BAD when they get wet?), so some have managed to 'mate'. They have a low survival rate because even the adults are prone to wandering off and getting lost.
You came close. However, Bigfoot pilots UFO”s. That fact is why no one ever finds a Bigfoot body; for when a Bigfoot dies on earth the remaining Bigfoot crew picks them up and whisks them back to Bolok for burial.