I’m pretty sure most young mammals engage in symbolic thinking when play fighting or play hunting. A branch can represent prey, two young can take the rolls of hunter and hunted etc. This helps them mimic their parents and practice hunting, fighting, exploring and social skills with the added bonus of diversion. Desmond Morris talks about this at length in most of his books…especially how human adults maintain many of these young features (neoteny), that is…playing make believe or pretending on object represents another one (note this is not refering to complex symbolic thinking let alone anything like writing). According to him, cats and dogs (just like humans) retain the qualities common amongst young mammals and also show neoteny as adults (while most other mammals grow out of this). Playing pretend is just one feature…dogs and cats also maintain child like features and both physically and in their behaviour. Fully grown cats and dogs can engage endlessly in play fights and play hunting and can even learn basic associations (or encounter them on their own). To a limited extent, cats and dogs definitely think symbolically…and likely more than most mammals…but it’s a pretty rudimentary kind of thinking in any case.