I’ve owned one of these for a couple of years now. I keep meaning to use it for plants, but It keeps getting hijacked for other uses. Last spring, I put it under my chick brooder to keep my baby chicks warm. Right now, it’s warming the mealworms that I feed to those same very spoiled chickens. I expect it would be a total cat magnet if I threw a towel over it and left it out for them to lie on. And it’s great for encouraging the seedlings of heat-loving plants to sprout more quickly.
Basically, this is a rectangular black plastic flexible mat that plugs into a standard electrical outlet. It feels like a pleasant sun-warmed surface if you put your hand on it, maybe 80 or 90 degrees Fahrenheit. If you put it on an insulated surface it’s warmer than on an uninsulated one. Because they’re designed for greenhouse use, they’re waterproof, though I’d still use care, especially with the electrical cord.
If I were putting it under animals (especially baby animals) I’d keep the cord and the mat where it couldn’t be chewed on. I’d also monitor the animals for over-heating and make sure there was a part of their cage that wasn’t on the heat mat so that they could self-regulate their temperature.
If you need a precise temperature, you can buy a thermostat control. I haven’t used the thermostat, so I can’t report on its reliability. These heat mats come on a variety of sizes, from 9″ x 9″ inches up to 20″ x 48″ inches, with the price increasing according to size.