Hello Oberon,
sometimes it does happen that the lower guardian spirit changes its shape to another animal. On the other hand, and especially in our very first months of meditations but also later on, it can also happen that iwe think of other things during meditation or are being influenced by things we have recently seen or heard, or by wishful thinking, and so there can be pictures in the meditation that do not belong there. (This is one of the reasons why personal supervision is so important in magical work, in order to help the student improve their ability of discrimination and to better understand their meditation results.)
To answer your question: it should be helpful to simply do some more meditations without expecting your lower guardian spirit to be of a particular shape, thereby enabling it to appear in the shape it fits best.
In Light,
Andrea