Un lion chassera une antilope

Bonjour John !

I think here you're mixing Le Passé Composé (or Futur Antérieur to be exact) and the passive voice :)

You are correct that in compound tenses, -er verbs use avoir, but here Le Passif is not a tense, but rather a perspective.

Indeed, if you look at the two sentences, the difference is who is doing the action:

Un lion chassera une antilope.   -> the lion is hunting.

Une antilope sera chassée par un lion.  -> the antilope is being hunted.

Now look at the sentence you suggested:

Une antilope aura chassé par un lion.  -> An antilope will have hunted by a lion.

-> Here the sentence has a complete different meaning and is actually incorrect :)

I hope that's helpful!
Bonne journée !