A Llanelli man claims to have come face to face with a feline-like animal "twice the size of an alsatian" while fishing at the River Towy near Llandeilo early on Wednesday.
His sighting has been backed by a farmer who says a stillborn calf on his farm was half-eaten a couple of weeks ago.
Forty-year-old fisherman Mark Richards said: "At first I thought it was a dog. It's unusual to see a dog loose on farmland, but this was twice the size of an alsatian, with a three-foot tail arching upwards and a bob on the end."
A local farmer told the Post he had spotted a large cat in the area a week ago. He said: "I've only seen it once, it was sitting on a wall on the farm. As soon as it saw me, it went."
A spokeswoman for Dyfed-Powys Police said: "We haven't had any recent sighting of large cats in the area, but if anyone does see something they should report it and we would advise they don't approach the animal."