It may be that there is a difference between glyphs and Glyphs. A glyph may be just a random coloration in the shape of the symbol of a Glyph, perhaps imparting some succeptability to that particular power, and a Glyph is a magical item created by Talad that has the shape embedded in a crystalling structure bound up with the power of the appropriate way. Thus your pet groffel might have a discoloration on his skin in the shape of a weakness Glyph and, say, if it was a darker color than the surrounding skin it would offer your pet some innate resistance to the weakness spell, if it was lighter the spell might be more effective when cast upon him. This is of course pure speculation on my part but it seems to make sense.