It all depends on "why the animal is thin". Are they ill, malnulturished, or just dont eat that much? Most ways for any type of animal to gain weight is to make sure they're healthy, and to make sure they're gettnig all they're essential vitamins and minerals (just like us humans!). If they're not eaty healthy foods, or not that much food. You may want to get them supplements for their vitamins and minerals. You can get them in pill form or powder. I find that powder is much easier to use as you can just mix it in with their food. If the fact is that they just don't show like they want to eat very much, trying putting something into it that will make them eat it. (table scrappings, juices from meats,poultry, or just grease from your cooking. you could also put some of their favorite treats into their food, but mix it all around so the taste is everywhere, you can also try gravey, although that can make certain digestive system's loose if you know what i mean!). If you're animals eat regularly, and seem pretty healthy you can always feed them (or mix in with their feed) fatty foods. Try canola oil, as that is healthy for their digestive system, and their shiney coats. It's also one of the most fattning thing you can consume. Mix it up there are endless things you can do to put weight on your animal. Some are just harder than others. The most important thing is making sure your pet is healthy, and if it's constant concern, or something seriously seems wrong, contact your vet with your concerns, it may be more serious than you though!