Something I've also seen recommended and is okay for humans but deadly to cats and dogs is TEA TREE OIL.
Tea Tree oil is ok to use externally on dogs (NOT on cats). It is an ingredient in many shampoos, ear cleaners, hot spot sprays, etc. The important thing to remember is that, like many medications and drugs, a little bit can be medicinal, but too much is a poison.
Here in the US, people seem to think that if a little bit is good, MORE is better. With things like Tea Tree, it's the opposite--the less you use, the better. I have used it in ear cleaners (for dogs only, never for cats) and shampoos. Basically the rule is that you don't want the dog licking it. If it's in a shampoo, it's rinsed off, and if in an ear cleaner, the dog can't lick it. If in a hot spot spray, make sure the spray also contains a bittering agent so that the dog will be discouraged from licking.
We're talking about a tiny amount of the stuff, like a few drops. I wouldn't use it straight on an animal, I'd dilute it down with alcohol, water, or a carrier oil. Yes, it can be absorbed through the skin, which is why it's very important to use a minimal amount.
Ironically, I don't take such care when I use it on myself--if I have a skin irritation or itchy spot from shaving or whatever, I dab it on straight. It really does work--it tingles, much like peppermint or eucalyptus, which helps take your mind off the irritation. It's antibacterial and antifungal too.
Cats should not come anywhere near Tea Tree, nor any other essential oil. Cats are extremely sensitive to essential oils, with the possible exception of Lavender in tiny amounts. If I want to use Lavender in my grooming shop for aromatherapy purposes, I put a little on a cotton ball and place it near the cat's cage.