We do not have fixed rules regarding what makes an acceptable entry for most of our competitions, other than the technical formats of the images, such as maximum pixel dimensions, colour space and titling rules, all of which are in the Annual Programme Booklet. The exceptions are The Don Harris Memorial Competition, where the entries must be in monochrome and Natural History, which has some special rules regarding the subjects, which are disseminated each time this competition is held.
The Portrait Competition is a case in point - there are no fixed rules here but we recommend that you take note of the following advice:

Whether the subject of the image is human or another type of animal, the photograph should attempt to convey the underlying character of that subject. This is entirely possible with humans, chimps, dogs, cats, horses and other large mammals etc., but will not be possible with smaller mammals or insects etc. where a lack of direct empathy may be experienced by the viewer. The subject should be the principal focus of the image and the features of the subject should dominate rather than any activity or the environment.

When entering a Portrait Competition, you have to accept the judge's personal and subjective opinion and if he or she believes, as some do, that the subject should be human, he or she will not enthuse about an animal subject, a risk the submitter must take. You should also note that non-human subjects are very unlikely to be chosen to be submitted by the club for external competitions, which tend to have stricter rules.