The time is 3 A.M. on a Saturday. Even now, on an ordinary day, there'd be at least one staff member or volunteer present at Sweet Friends Animal Shelter to keep tabs on all the dogs and cats in their care. Recently, however, they've been short staffed, and a certain deer noticed a Furbook post mentioning such.
David pries open a window adjacent to the shelter's door. He knows, of course, that it's not well-funded enough to really have much security, not even cameras, but it feels a bit more secure. His heart is racing.
The shelter is split into two wings, one for cats and one for dogs, with each housing a little under twenty animals each.
GRRRWWWWLLL
The deer is certainly quite hungry, but while the dogs will assuredly be more filling, the cats will be great appetizers. He enters the cat wing as silently as possible. A couple of the animals stir from their sleep and glance up at him with piercing eyes as he enters. 
He stops at the first kennel with an awake cat, a tabby that stares at him with deep curiosity. He undoes the latch as silently as possible and gently reaches for the cat. It scoots into the back of its cage, glaring at him now. Deciding he needs to get it in him as quick as possible, he sticks his head in alongside his hands and stuffs it down his gullet in one quick motion.
GULP GLRRGLE
He huffs a little as it squirms furiously inside of him, its shrieks muffled enough by his stomach to not cause too much disturbance. He bends down to a siamese cat in the kennel below, which is eyeing his gut with fear. He grabs the cat out and shoves it into his cock just as quickly as he'd eaten the previous.
SHLLRK GLORP
With his gut and balls both writhing with one feline each, he goes down the line. A black cat for his belly, a calico for his cock. A sphynx meets his stomach, a persian for his penis. The room is soon alive with yowling, frightened cats, and a couple dogs whine in aggitation from the other wing. The hissing and shrieking and meowing grows quieter and quieter as David reallocates more and more of the noisy animals to his insides.
GULP SHLLRP ULP SHLORP
Finally, with 17 cats squirming inside of his body, he reaches the final one that the shelter currently has on hand. This one is a long-haired white cat named Snowman. David grins as he pulls it out of its kennel. It doesn't struggle, only stares in defeated terror.
The deer turns around and holds the cat in place behind him as he bends over. With only a moment's hesitation, he stuffs the terrified cat where the sun doesn't shine, reveling in the feeling of it squirming against his prostate.
SHLRKKLRBLT
He walks out of the cat wing with a yowling, squirming gut and a writhing, hissing sack.
The dog wing is his main course, and he's all too eager for it. A good few of the dogs are awake already, but some managed to sleep through the chaos on the other side of the building. David decides his best bet is to start with the awake dogs to keep them from starting a bark fest while they're still outside of him; it wouldn't stop him, of course, but it would be irritating.
The deer reaches for an awake Chihuahua first. Such a small dog goes down easily. Barking joins the meowing in his gut.
ULP
15 more, 5 of which are awake.
He grabs a Beagle next. It's easy enough to stuff into his cock. He doesn't want to completely fill his balls just yet, though, so he slides it into his bladder instead.
SHLRK SLUSH
He's getting very, very full now, but he's not satisfied just yet. The next dog is a Labrador, which he gets into his gut with some difficulty. Then it's a Terrier for his bladder, a Poodle up his ass, and a Collie for his nuts.
GRRRRGLLE SLOSH CHURN SHLRRK SLUSH
Though a few dogs have roused and are shuffling about in their kennels not fully awake, there's one last dog that had awoken during the cat incident: a husky. The dog stares at him with blue eyes that almost challenge him. David knows full well the kind of meltdown the dog is about to have. 
He attempts to clamp his hand on the dog's muzzle before it can start flipping out, but it manages a bark beforehand, and soon the barking isn't just restricted to his gut anymore. With a sigh, David stuffs the husky's head into his maw. The barking outside of him doesn't seem like much, though, with all the commotion of 23 animals inside of him.
GULP ULK SLRP
David slurps down the dog and turns to look at the remaining 10. It's not easy to maneuver at this point, being so full, but he just can't help himself. He needs more. Just 10 more.
The next 9 are a slow process, opening each cage awkwardly from the side and wrangling the dog within with one hand before stuffing it wherever he feels needs a bit more squirming and barking. Finally, he's left with one last critter to claim, a German Shepard he's been saving for last.
Its tag claims it to be well-trained, and David decides he wants to test this theory. He opens the door and points to the ground just behind him. "Here."
Shaking in every limb, staring at him, the dog does as it's commanded and moves to that spot. "Sit." The dog obliges, hoping its obedience will earn it mercy.
David stands fully and staggers a pace back, his ass looming over the sitting dog. He plops down with the weight of himself and 34 meals directly onto the poor animal, stuffing it deep into his colon. He lets out a satisfied moan and takes some time to enjoy all the squirming and yowling and barking in his own gut while his donut hole slurps up the german shepard's legs and tail.
BARK SLOSH WOOF WOOF YEOOOOWLLL GRRRRRRGLE MEOW HISSSSSSSS CHURN ARF ARF RRREOOOOOW GROOOAAAN MEEOOOWW BARK BARK GLLLRRRGLE RRRRGGGLLG RUFF RUFF RUFF HISSSSS SLOSH SPLORSH CHUUUURN
The over-stuffed deer nearly passes out from the pleasure he's feeling from his entire body stuffed with the would-be pets. Finally, though, he gets up. He knows what he needs to do now.
He waddles out of the building with the entire animal shelter's former population still writhing inside of him and makes his way to his car. A rental moving trailer is attached to the back, which he makes his way into. There, he rests as the struggles and the meowing and the barking gradually die down inside of him and his guts and balls smooth out, going from messes of paw imprints and poking animal snouts to quite round shapes, albeit with bits of bone creating defined bumps in plenty of spots. 
In the trailer with him is a rather large aquarium. By pushing it out onto the ground below the trailer, he gets enough height on it to begin emptying himself, starting with his cock.
PLAP PLAP PLAP SPLRRRRTGLRL FFSSSHHHSSSHHHHSHHSSSSSSSSS SPLOOSH SPLASH
After ejaculating the contents of his incredibly full balls and releasing the mess of piss and bones from his bladder, he's already filled the tank with gallons of bodily fluids and the skeletons of a dozen animals. "Huff... Huff... Huff..."
With his body a bit lighter, he's then able to hang his ass out over the top of the tank, holding on tight to the trailer to keep himself steady. Then...
FBLLRLBRLRLBRLLL SPLASH PLOP SPLISH SPLAT PFFRRRRT FLLRBLRBLRRRRT SPLASH FBLLRLBLL BLOOSH
The German Shepard's skeleton, encased in a fat log of shit, is the first to come out. The Poodle and formerly-white-but-now-brown cat follow shortly after. Then the rest of the animal shelter vacates his bowels, all splashing into the macabre display.
When he's finally done, David puts an air-tight lid on the nearly-full tank of mingled animal bones and bodily fluids and sits on the edge of the trailer to admire his handiwork and masturbate again while staring at it, opting to blow his load onto the parking lot's asphalt.
His hips are much thicker now, his belly much larger, and his entire body that nice bit chubbier. He admires the additions for a moment, then checks the time and realizes he needs to be out of there in a few minutes to give himself the 2 hour window of leeway he planned before opening time.
The deer prepares to leave. He loads the aquarium into his trailer, leaves a nice note on the door, and hurriedly gets in his car, needing to adjust the seat to accommodate his added mass. He drives very carefully on the way home, his greatest concern keeping his new home decor in-tact.
Not long after, the first workers will arrive to an empty shelter. They'll find a note on the door saying "Thanks for the buffet!" and an anonymous review online reading "5 stars, my compliments to the chef. Best meal I ever had."