Alternate explanations more plausible than "real bigfoot":
1) You've made the whole thing up.
2) You did see the thermal image in question, but were mistaken about its size and appearance. It was, in fact, a native wildlife species such as a bear or the hind end of a moose.
3) You did see the thermal image in question, but were mistaken about its size and appearance. It was, in fact, a human being accidentally captured.
4) You did see the thermal image in question, but were mistaken about its size and appearance. It was, in fact, one or more human beings intentionally pranking you.
Every one of these alternate explanations is vastly more likely than that you momentarily viewed a real "bigfoot" through your device. You've provided nothing to rule out any of these possibilities.