How is fehlen "missed" in this instance?
It's a participle (or gerund?) so that it is like an adjective. The present tense is taken by the "are".
what exactly is the subject here?
'Wir' is the subject, 'fehlen' is the verb, 'den Tieren' is a dative object.
Is this like a passive voice verb?
It's not actually passive, because its intransitive. But it is like gefallen.
Is there no reason why den Tieren must be a dative object other than fehlen requires it?
There's an implied to/by, if that's the sort of reason you want?
Yeah I suppose so, thanks.