You have a false premise and your conclusion doesn't follow from your premises. For our purposes, UFOs being real means they are of extraterrestrial origin.

P1: Many people claim to have seen UFOs and are not believed. True

P2: Many people claim to have seen Bigfoot and are not believed. True

P3: The United States government says that UFOs are real. False

C1: If the United States government says that UFOs are real, then UFOs are real. False

C2: If UFOs are real, then the UFO sightings must be legitimate. False

C3: If the UFO sightings are legitimate, then the Bigfoot sightings must be legitimate. False

What happens when you apply this to other cryptozoological phenomena? You have just proved the existence of the Loch Ness Monster, mermaids, ghosts and chupacabras.

You have a false premise and your conclusion doesn't follow from your premises. For our purposes, UFOs being real means they are of extraterrestrial origin.

P1: Many people claim to have seen UFOs and are not believed. True

P2: Many people claim to have seen Bigfoot and are not believed. True

P3: The United States governments says that UFOs are real. False

C1: If the United States government says that UFOs are real, then UFOs are real. False

C2: If UFOs are real, then the UFO sightings must be legitimate. False

C3: If the UFO sightings are legitimate, then the Bigfoot sightings must be legitimate. False

What happens when you apply this to other cryptozoological phenomena? You have just proved the existence of the Loch Ness Monster, mermaids, ghosts and chupacabras.