Putting this in here, rather than tagging project sticky:

Definition of interspecies

Wiki for interspecies says:

They should always be from different family sub-classes (e.g., feline/canine, equine/cetacean, but not lion/tiger). A good rule of thumb is if they can realistically breed offspring this should not be used.

i.e.

post #936128 - interspecies

post #936233 - Not interspecies

If this is true (the bottom example is currently tagged as interspecies anyway),

Then should there be a specific tag for inter-..species scenes?

Such as [lion x tiger], [dog x fox], [horse x zebra] etc.