(Not sure if it qualifies as "facesitting" when the butt is large enough to smother the entire body, not just the face.)

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All credit for the art goes to PrinceBlueMoon3 as the artist, and Astrum as the author of the fanfic, which was first posted in the comments section of >>2214867

All I did was make the original comment that indirectly inspired this whole thing, and upload these pictures of the adaptation.

Silverstream lit up then, and squeezed him tighter for a moment before then gently letting him go so their sides could separate again and the two turned to look at each other. Her eyes once again met his with that seductive glare and smirk. “Hmmm, well then, I’d say it’s time you check my rump too. What do you say?…”, she cooed devilishly. “W-whoa, wait, wait, take it easy, Silverstream, I’m not-“, Gallus was cut off before he could prevent her from forward and standing directly in front of him. The ground seemed to shake again as she planted herself directly in front of Gallus, presenting her massive rear to him in all its grandeur. Just as soon as he had started to gain his bearings, Gallus had lost his composure again at the presence of this new position. The large orchid globes loomed over him, her light blue tail dangling in between the two swollen cheeks. Silverstream craned her head back to gaze into Gallus, those lidded, deep purple eyes beckoning him to worship her form. “U-uh…Silverstream…”, Gallus attempted another objection. But before he could get it out, Silverstream suddenly backed up and pressed her ginormous ass into his face. “Mmph!”, the griffon yelped in shock under hundreds of pounds of hippogriff fat. “Oooo~…”, Silverstream moaned as she drove all of her girth onto Gallus, her swollen cheeks swallowing up his whole front body. He reflexively placed his claws square on both as it startled him.

"I won't post the rest of it because it gets a little steamy per say. And I think both Silverstream and Gallus are already steaming so I don't want to stoke the flames of wrath any further. You can find the rest of the dialogue her if you're interested. It's in the comment section by Astrum." ~ PrinceBlueMoon3