Fake mobsters in suits and spats are pretentious. But a horse head in your bed? That's portentous. And also, call the police.
A regular person who insists on using the royal "we" is pretentious. Find the word "pretend" in pretentious and you're onto something — both words come from the same Latin root. Pretending is fun, but anything pretentious is a bummer because it's fanciful and untrue. Examples:
"Eva's colleagues eventually turn on her and deem her uppity and pretentious; one day, someone in the office steals the diary." (The New Yorker)
"Well, the film was a bore, with far too many pretentious slow-motion surfing shots or close-ups of foam and bubbles." (Los Angeles Times)
Portentous is darker. The word comes from the Latin portentosus for "marvelous, threatening," oh, and also "monster." A funnel cloud, a horse head, or a letter from an ex, are all portentous signs. It overlaps with pretentious because portentous also describes someone trying hard to seem important, like a human pufferfish. But it's usually a sign of things to come, like a portent, or omen. Observe:
"But above them all looms one legendary beast: the great white whale, Moby-Dick, freighted with portentous doom." (The Guardian)
"Some are table-centered domestic scenes: silent intimacies, fights over a portentous envelope." (The New Yorker)
If you get them mixed up, remember that pretentious has an "i" at the end, and portentous has an "o" for "omen."