In modernspeak, 'don't look for the Hallmark symbol+ on the back.' A horse's health can be determined by a quick look at the quality of its mouth. A horse with healthy gums and minty fresh breath is probably doing pretty well, whereas a horse with gingivitis and tooth decay probably has other problems.

What this means is to be happy you got anything at all (see also beggers can't be choosers). If you spend time looking for the faults of a gift, you won't enjoy the gift as it was meant to be enjoyed. However, if the horse is made of wood, you should cross-reference against other sayings.