Danny McBride could be in for a crappy surprise -- literally -- over the use apparent use of real monkeys in his hit show "The Righteous Gemstones."

PETA tells TMZ ... the org is threatening to send McBride fake monkey poop after spotting what they claim is a real capuchin monkey in the show's trailer.

PETA claims they have been firing off emails to McBride's reps and HBO execs demanding primates be left out of the show, and a commitment to never using wild animals in productions again ... but haven't heard back.

Now, the animal rights group says they'll be watching the upcoming episode on Sunday -- and if a live monkey appears, this fake pile of doo-doo is going to show up at the front door of McBride's production company.

Debbie Metzler, PETA's Senior Director of Captive Wildlife, tells us ... “Danny McBride has all the advanced technology at his fingertips, but he chose to further exploit a suffering monkey who should be in a rainforest, not on a TV set.”

The organization is urging viewers to boycott the "shameful show" in protest and calling on McBride to reevaluate his "crappy" choices -- pushing him to use CGI in future projects.