Photographing unusually dark or unusually light colored subjects is difficult because your camera's metering system is being "fooled". It's always looking for "18% gray", so when something is darker than that, it will tend to want to over-expose (something lighter than "average gray" it will want to under-expose).
If using any of the auto exposure modes, you need to override the camera's tendencies with Exposure Compensation... using some minus E.C. with black, or some plus E.C with white subjects. If shooting manual exposure (NOT manual with Auto ISO, which is another auto exposure mode), you need to make similar compensation.
I agree with the suggestion to shoot RAW... that will give you more latitude to adjust image brightness later in post-processing (which is best done on a calibrated computer monitor... most are overly bright when not calibrated, causing you to make your images too dark).
Also try to make the photo in shaded or overcast lighting, rather than bright, full sun.
Very few "black" critters are actually black. With black, some sort of reflector or fill flash can help open up shadows and find detail, too. In this case, the horse was standing near a white building and the arena sand tends to be high reflective...
Spot metering is not necessarily a good thing. If you do that, you will need to more precisely control Exposure Compensation (or manual exposure override). In fact, a wider metering method usually is more forgiving, when the tonalities throughout the scene average out to around 18% gray.... Both the images below were metered more broadly and actually shot with some + E.C. dialed in (tho a little less with the black horse than with the white).
Both the above venues I know very well from having shot there many times over the years.... so I know what works and what doesn't.
I also carry a separate, incident meter and use it in some of the trickier lighting situations. This type of meter measures the light falling onto a subject, rather than what's being reflected off of it. For that reason, incident meters aren't "fooled" by different subject tonalities. (It also serves as a flash meter, when needed.)
When out in full sun, that's often the trickiest of all. If possible, use some fill flash (most horses ignore it)...
Of course, flash isn't always possible, practical or even allowed. In those cases all you can do is keep shooting, then make adjustments later in post-processing. That's much more easily done if you shoot RAW files instead of JPEGs. But you need to have an image editing software and calibrated computer monitor to work with....