Well, of course, the guineapig was used sneeringly. But actually it could be seen as quite a compliment as guineapigs are charming creatures who are ideal pets because they are peaceful and don't bite. That leaves snakes in the grass, sharks, and animals with similarly threatening images, to be used for other deserving politicians.
As usual, there is an inconsistency of outrage. Oneof the anti-Corbyn group quite recently said "I can smell a bubbling cauldron again" - but since it was addressed to posters on the left, no issue was made of it.