Clearly, a dog's consent wouldn't have a form of human consent. Elliot Svensson's answer goes in such direction. Humanizing non-humans is a common but wrong subjective interpretation of causal mechanisms (actions, reactions) of other entities, like animals. I'm not telling his answer is wrong: consentment would only properly be applied to human interaction. But perhaps we can do an interpretation.
Perhaps a systemic approach regarding interaction can be applied. Interaction produces attraction or rejection as a result (e.g. you've bought delicious bread from bakery X; then, you experience some attraction to such bakery, which would cause you to come back; if you didn't liked it, you will try to get far from it). So, if the animal feels attracted to the interaction to be performed, we can assume such causal reaction as a form of consent, without humanizing the animal actions and reactions. This is one issue to consider.
The issue here is that such reaction (attraction or rejection) is produced after the interaction. Consentment is usually associated to an interrogation prior to the interaction. So, since we cannot ask a dog if he would like to test a drug, just give it to him, consentment wouldn't be possible. This is the second issue to consider.
Third issue: in the second case, since animals can make associations, Pavlov ringing the bell could be associated with proposing the action. If the animal has a positive (attractive) response, it can be considered as consentment. But here, we're already assuming that ringing the bell is trigering a form of understanding, so, again, we're humanizing the process. Kant wouldn't agree.