So, for some time I have been playing a cleric for Tyr and an idea accured to me. But for this idea to work I need to know what plane celestial animals come from, specificly celestial dogs. I'de think it would be Arvandor since it really is part of the celestial planes, but what I have read on Arvandor doesn't say anything about celestial animals. However I read that The House of Nature's inhabidants become celestial creatures, even though the House of Nature is a Neutral plane. So, does any one know for sure what plane celestial animals come from, since I'de imagine he'd know. Or where my character goes IC to find out?

That knowledge for a PC to have would involve some intensive study and pursuit of knowledge--likely over a long period of time. That knowledge is not well known to PCs, NPCs, or anyone outside the other planes really, and is something I would consider extremely rare knowledge.

PCs would know that the Gods live somewhere, in another plane. They may (may--but probably not unless they have a lot of lore points) know the name of that plane. As a general low level cleric you would likely know that the creatures are from another plane, and that due to your faith and prayers your God sent them to help you out of a tough bind. The information in all those source books is really behind the scenes stuff that no PC would likely ever have a chance of knowing.

Okay, so how then, do I summon creatures of my alignment? I've so far seen only True Neutral animals when I use summon creature. As you said the summons are god sent, wouldn't that mean that clerics would summon creatures based on their alignment/faith?

Sometimes that is true, yes. We do have some dieties set up with special sets of summoning themes (cleric of Grumber, for example, would summon earth based themes). It's some scripting work though to put them in for all dieties, so some just use the basic underdark theme. Your God is still sending you help in response to your prayers.