Black pepper oil is an oil to be extremely careful with. As to be expected of black pepper, it is an irritant when experienced in concentration. However, it is not toxic - only irritating.
It can be used in massage and is a great muscle relaxant. I've read that it is a good oil to use on older horses - not necessarily in aromatherapy, although I have met one horse who couldn't get enough of the smell of it. It is good for these older horses because it helps ease arthritis and rheumatism, muscle aches and pains, and is a circulatory stimulant, which helps with stiffness. See what I mean about it helping older horses? Does that sound like any elderly horses you know? Or for that matter, maybe some elderly people?
It stimulates the appetite, aids digestion, and can relieve colic symptoms.
It can be used for mild pain relief and promotes general health.
For horses of any age, it can be used with muscle sprains/strains after any kind of exercise.
These traits were all taken from Essential Oils for Horses by Carole Faith.
One remedy that I read about last summer when researching things to treat Orion's legs with used black pepper, lime, eucalyptus, and peppermint oils mixed into a base of sweet almond oil (which, by the way, is the base I use to mix my oils in, and Orion doesn't mind it). The oils were mixed with 3 drops of eucalyptus, and 1 drop each of the lime, peppermint, and black pepper oils in 10 mL of sweet almond oil. It was massaged into the legs to treat overworked tendons or muscles.