A few nights ago, I attempted to summon a familiar, today I began searching for my possible familiar in a park (someone told me to look for the familiar). I decided to give up, until I was just standing in front of a bush, eating, and a rat (or mouse, but it looked more like a rat) scurried out in front of me, looked up at me, I think I must have flinched because it scurried away after a few heartbeats of starring at me. The only features I remember was it had golden-brown fur and blue eyes (from research I just did, I don't actually think that's possible).
I didn't see it come out from the bush, and when I inspected its hiding place, it was gone. as if it just vanished into thin air.
The blue eyes is what hit me, last night, I dreamed of being watched by a pair of glowing blue eyes, and the rats eyes must've been noticeable, considering I was only looking at it for a few moments. (I would also like to mention the rat looked fairly healthy, I feel like that was an important fact for some reason).
In the dream, the blue eyes turned into a patch of stars, then the dream ended.
I don't know if this mouse was my familiar, because a mouse seems like an unlikely partner for me, I was expecting more of a cat or wolf kind of thing. But hey, it was a cute rat!.
And that brings me to my second question, can familiars change forms? From my research, it is possible, but what do you think?
I suppose the first thought is that the familiar that comes to you tends to more often be the one you need over the one you want. There is more to them than surface appearance.
I would suggest looking up some of the meanings and interpretatioms of rats among different cultures. For example rat in the chinese horoscope an be interesting and might give some information to ponder over. Same for the symbolism for rat within the tradition you follow, and perhaps other shamanistic/animistic traditions. Or even in dream symbol books if you have one. There tends to be a lot of commonality between sources. Animal-speak, by Ted andrews has a pretty good glossary at the end for different animals and their meanings.
I do agree with the thought that some entities can indeed make changes to their appearance, or at least in how they represent themselves to you. Consider that they are beings of energy and consciousness. Energy itself is by nature formless. However the nature of that energy also still trnds to have the same qualities in general. So even if a guide or other entity chooses to change its shape or appearance it would still feel the same when present.
Personally though, I find that it isn't that common for a guide to change too drastically even if they can. They are usually pretty purpose-driven and so would only deliberately change themselves if it would serve you or the overall relationship in some way. Sich as using the form as a message itself through what it represents symbolically or even in behavior.
I kind of don't have much to add, since I think Spirit said pretty much what I would say, however I think, though that it is possible for familiars to change their shape, I don't think they'd do it unless Absolutely necessary . Its like us humans changing our hair or clothing, We Don't have to do it, unless we want to or if its needed.
A summoned spirit isn't inherently a familiar unless such an agreement is made. One cannot summon a spirit that is already their familiar right off the bat. There are always misconceptions floating around about familiars, but this seems to be the most common. You summoned a spirit, but that spirit was not your familiar. Coming to you in your dream may have been its way of saying it agrees to be your familiar since that was your petition, however. Just keep in mind, you cannot summon something and automatically expect it will be your familiar. Open invitation style conjuration can end poorly. It's best to know exactly what you're bringing up, lest you invite something you can't put back down. Building a relationship with this spirit before doing any workings with it is advisable. Not everything is always as it seems, and a seemingly innocent looking rodent may come bearing gifts of plague, such as it were.
Just a tip for the future in the occult. If you don't actually understand how something works, you probably shouldn't attempt it until you do. You can end up in some less than pleasant situations that way.