Awww I know how it is-I still want my pony

Parents hey? *eyeroll*
Maybe they'll let you foster-to get their feet wet? Or pet sit?
We're pulling for you
Oh and this totally depends on your relationship with your parents and how old you are and stuff; but if your mom did/said something to make you think she was going to say yes to this dog-I'd call her on it. (But that's my family dynamic not yours-so use your sense)
I would say "you know mom? That was really disappointing. You said you would consider the dog, and the way you said it made me think you were finally going to allow me a dog. And when you went back on your word, it was upsetting and disappointing. So whether you are going to allow me a dog in the future or not-could you at LEAST be honest with me?"
I think that way your not attacking but talking about your feelings and really making her think. Like I said-may not be appropriate to your situation thou