Some users reported that the helicopter or Bigfoot stops moving on the chase sequence. The only recourse is to reboot and replay the Northwest scene.
He is in the Pacific Northwest, where he lives in a cave. Once you have confirmed the other 3 cryptids, you can go there and follow him in the helicopter.
You don't. However, you will later follow Bigfoot as he travels through the trees in the Pacific Northwest.
In the Pacific Northwest, follow Bigfoot in the copter as he moves through the trees. Be careful because he may trick you. You have to follow him until he reaches the caves.
For some few users, either Bigfoot or the Helicopter fail to move at the Pacific Northwest. This glitch was much more common when the island first came out. You can try rebooting Poptropica or playing another island until your browser cache is cleared out, then return to the Northwest.
Once you have taken a photo of the footprint in the Himalayas, and proven the other three cryptids (Nessie, the chupacabra, and the Jersey Devil), Mews will send you in pursuit of Bigfoot (a similar creature to the Yeti) in the Pacific Northwest.
You cannot prove the Yeti. But after you confirm the other three cryptids (Nessie, Chupacabra, and Jersey Devil), Mews will send you in pursuit of the similar creature, Bigfoot, in the Pacific Northwest. The location will be added to the GPS on the helicopter.
When you have confirmed the existence of three cryptids (all but the Yeti), the final location is the Pacific Northwest. You will go there to follow Bigfoot back to his cave home. But unfortunately, you will also have to save him from Gretchen Grimlock. (see related question)
When you catch Gretchen Grimlock's helicopter, jump onto it from yours, then click her gas cap and fuel will start coming out. Cut loose the line to Bigfoot's cage with your gardening shears, and you both float safely down on the cage's parachute.
Bigfoot lives in a cave in the Pacific Northwest, and you cannot go there right away. You only locate Bigfoot after you have visited all four of the original sites (Loch Ness, Himalayas, Puerto Rico and New Jersey) and proved the existence of three cryptids -- Nessie, Chupacabra, and Jersey Devil. When you have gotten the footprint photo from the Himalayas, and confirmed the cryptids at the other three sites, Mews will tell you that Bigfoot has been sighted in the Pacific Northwest, and you fly there to find him and track down his home.
You can try rebooting Poptropica or playing another island until your browser cache is cleared out, then return to the Northwest.
You cannot prove the Yeti based on the evidence from the Himalayas. So you have to prove the other 3 (Nessie, Chupacabra, and New Jersey Devil) and when you do, Mews will send you to pursue Bigfoot in the Pacific Northwest.
He is not on Cryptids Island. He is on Counterfeit Island.