I found it on Project Gutenberg.
It is The Drug by C.C. MacApp, available on Project Gutenberg here.
They are testing a hallucinogen on a litter of pigs.
The pigs were in pens outside the lab. Amos had seen figures on weight gain and general health (the latter was what promised to be sensational) but hadn't seen the animals for two weeks. He eyed the first bunch. "How old is that boar pig?" "Not quite four months." Amos was no expert, but he'd spent many hours on customers' farms and he thought the animal looked more mature than that. So did the shoats in the same pen, though they tended more to fat. All of the group had an odd look, certainly not normal for Yorkshires of their age. He thought of wild hogs. "Is it just the general health factor?" he asked. "I don't think so, Mr. Parry. You remember I told you this wasn't actually a hormone." "I know. You wanted to call it that for secrecy, you told me." "Yes, sir, but I didn't tell you what it really was. Mr. Parry, are you familiar with hypnotics? Mescaline, especially?" "No, I'm not, Frank." "Well, it's a drug that causes strong hallucinations. This is a chemical derivative of it." Amos grinned again. "Pipe dreams for hogs?"
Here is the pig trying to look nd act like a black cat.
This one was isolated in its own pen, and it looked even stranger than its siblings. In the first place, its hair was thicker, and black. There was an oddness in its shape and a vaguely familiar sinuousness in the way it moved that made Amos' skin prickle. "What's wrong with it?" he asked. "It's healthy except for the way it looks and acts." "Same litter and dosage?" "Yes, sir—all of them got just one dose. The effects seem to be permanent." They were leaning over the fence and the animal was looking up at them. There was an oddity in its eyes; not intelligence exactly, but something unpiglike. Abruptly, it stood up on its hind legs, putting its forefeet against the fence and raising its head toward them. It squealed as if begging for attention. Amos knew that pigs made affectionate pets. Drawn to it as well as repelled, he reached down and patted it, and the squealing stopped. It was standing too easily in that position, and suddenly Amos recognized what was familiar about it. He jerked his hand away, feeling a strong desire for soap and water. "How long's it been this way?" "It's changed fast in the last week." Amos looked toward the doorway of the lab, just inside of which a large black tomcat sat watching them. "Is the cat out here a lot?" Barnes' eyes went to the cat, widened, and turned back to the pig.
The man who is leaving the company tells his immediate boss he took some of the drug and feels great.Gives him information about the tests in a folder.
Barnes stopped talking and Amos said mechanically, "You've been part of the team for a long time, Frank. It's especially awkward to lose you just now." It was banal, but it didn't matter; he wasn't going to change the man's mind anyway. He looked closer. The timidity was gone. So were the eyeglasses. A frightening thought struck him. "You've taken some of that drug." Barnes grinned and handed a small vial full of powder across the desk, along with a file folder. "Last night," he said. "Between frustration with the job and curiosity about this stuff, I yielded to temptation." Amos took the vial and folder. "What are these for?" "So you can destroy them if you want to. I've doctored up the lab records to make the whole thing look like a false alarm. You're holding all that's left of the whole program." Amos looked for signs of irrationality and saw none. "Do you feel all right?" "Better than you can imagine. But let me tell you what you're up against. I can at least do that for you, Mr. Parry." "Thanks. Don't you suppose you could call me Amos now?" "Sure, Amos. First of all, you were right about that pig trying to imitate the cat. He couldn't do much because he only had a pig's brain to work with.
The immediate boss thinks his friend is dead then the black cat speaks to him. It is his friend.
"This whole thing's a delusion," said Amos dully, aloud. "No, it isn't," said a peculiar voice behind him. He whirled and saw the black tomcat grinning up at him. He gasped, wondering if he were completely insane, but in a flash understanding came. "Frank!" "Well, don't act so surprised. I can tell that you took some yourself." "Yes—but how—" "I thought it would be an easy life and I want to stay around here and watch things for a while. It ought to be fun." "But how?" "I anesthetized the cat and grew a bridge into his skull. It took five hours to transfer the bulk of my personality. It's odd, but it blended right in with his." "But—your speech!" "I've made some changes. I'm omnivorous now, too, not just carnivorous—or will be in a few more hours. I can go into the hills and live on grass, or grow back into a man, or whatever I like." Amos consulted his own inwardness again. "Is this possible? Can a human mind be compressed into a cat's brain?" "Sure," said Unconscious, "if you're willing to junk all the excess." He thought about it.
