Okay, first note that ANY dreams you will have while pregnant tend to be a thousand percent more freaky and distressing than normal. That being said...

My first impression was the old "everything is you in your dream" rule. SO I thought maybe this horse was a personification of the animal/primal part of yourself. Being black, it might be something linked to part of you that you don't like and have since been ignoring (hence the need for nurturing). It all depends on what the color black means to you. If you see it as signifying something bad, then the horse is part of your shadow self (the things about you you don't like). If it's strong, it may be related to strengeths within yourself that you are ignoring.

When you leave the room and go searching for the new brush, the meaning of the horse takes on a very different label...especially because you mention that you are pregnant. The horse now represents something new in your life that you have to nurture and care for (like the new baby). Your mother being hurt by this new addition could represent two things. First, the obvious anxiety over the understanding that you will have to be the mother now, and your mom will not necessarily be there for you all the time anymore (a common "I have to grow up" theme). Second, your mother may represent some parts of you that you fear may have to end with the arrival of the new baby. Either way, you are preparing to mourn the loss of someone/some part of your life in exchange for this new life coming in to the world. Don't get too upset, I've heard of much freakier dreams that were far more disturbing than this