Consent is a normal part of human sexual relations, but not necessarily for that of animals. Sexual coercion has been observed, sometimes as a social strategy, in many mammal species, such as dolphins and other primates. This does not justify a forceful sexual assault of an animal, but it shows that we should not expect to establish consent in the same way that we do for other humans.
This said, animals can, in fact, show consent. A healthy, mobile animal is capable of resisting unwanted touch and encouraging pleasurable petting. Animals even initiate sex acts with humans, often in the form of a male animal advancing on a person to thrust his pelvis against the person.
We do not require consent for slaughtering, enslaving, or breeding animals, thus the human standard does not and cannot always legally apply.