Ouch, I know that feeling! I think D is not meaning to offend or hurt you in any way. You said B is a new student, right? Well, maybe D feels inclined to welcome her. See, when a new girl came to my school last year (this was before I became homeschooled again), the counselor chose me to guide her around. Even after I wasn't required to, I stuck by her side because it was a whole new environment for her. Here is this girl, a city kid, used to giant schools, thrown into a school of fewer than 40 kids for 7-12. Less than 100 for K-12. It was the least I could do to make her feel welcome, and then we became good friends.

So what I think what D is doing is just making B feel comfortable. Maybe they are starting a friendship, but that does not mean you are not valued. You will always be valued by us, and I know by your friend, D, as well. Don't loose hope, and maybe talk to D about feeling upset? I promise you D will try to understand and will do everything in his/her power to make you feel better and include you again. Don't fret, my friend!

Okay, now let's see what I can do about part 2 of your situation... Something is queer about P, no offense. Seriously, I have some weird friends too. But a dog? Why a dog? So, okay, this leads me to think about something - usually if someone obsesses about an inanimate object or an animal over their own friends, something often is wrong. Like, in their life maybe they are upset or struggling with something? Maybe they lost a pet dear to them or fighting with family, or maybe feel left out by friends. My first suggestion, even if it is uncomfortable, is to maybe ask P if everything is okay. Don't ask her in public, wait until you two are alone so she won't feel pressured. If P asks what makes you wonder, be honest and kind. 

People say I think differently, and maybe I do. Most people would not look at this situation and think, "Oh, P must be struggling with something in her life!". But hey, I am different. Maybe different is good, and maybe it will help you find the issue behind this all. If all else fails, and your friend is indeed okay (for your second situation here), then remember that people do grow and change. Maybe she has moved on, as much as it pains me to say this. I wish you the best of luck, my friend, and do not lose hope! Even if friendships fall, new ones will always arise, and we will always be here for you, that much I know. 

To clear #2 up, C is the one who only talks about P's dog. C said"I think I'm more obsessed with your dog than with you." a few days ago, and the next day, C wrote an apology letter to P, which, according to P, only said 'really sorry' over and over, and didn't even say what C was sorry for doing.