We keep it vague on purpose so we don't have battles over the specifics of what makes a character not MLP related. It is best to take it case by case. For example, we have had images of Dr. Wolf reported for deletion, even old images, because it is a wolf OC. By technicality, it isn't MLP by design as there are no anthro wolf things in MLP. We allow it because the OC is prominently displayed with other OCs that are pony, the person behind the OC makes MLP content, and there is a historical component.
This is why a set rule to define this will not work. You can't really quantify when to draw the line. It is best to gauge if the OC deviates from this pattern though by being new, not being featured with pony OCs, has never been a pony OC, clearly is not a creature from the show, and does not make pony content. To define how many times it needs to be featured before it is "grandfathered" in would be too time consuming to be made fairly. Gut feeling works nine times out of ten anyway. Random ass dragons, humans, and cats that are not assigned an OC name and are not with a pony are typically trashed. I don't see how OC rules would differ. I would need a more specific example. Random cartoon horses that are clearly related to ponies in style or through parody are allowed. A horse from, just for example, some Cartoon Network show like Adventure Time clearly doesn't count unless it is referencing MLP. Anon is allowed under the same context. If it is just an anon that can't be interpreted as interact with ponies or probably located within Equestria, it is probably not going to stay.
This is why the rule states "Keep uploaded images relevant to My Little Pony and its fandom". It is pretty vague, yes, but it is also easily applied. It is also best to tag things that are not to your liking as well to filter it in the event that the OC in question is determined to be close enough to be allowed. For example, Verity is not technically MLP, but is related through being /mlp/ related. If by some disaster you come to hate such a cute pony and the artwork around her, using a filter helps to keep it both up as related to the fandom but out of mind should that be an issue for you personally. As for the report system, we will change that eventually, but it is lower priority to fix right now.
TL;DR report what you think violates the rules.