I personally think animals have their own kind of enlightenment and happiness, maybe even cessation because of lack of defilements, they can sit still for long without thinking. Have other teachers say it as well.
Because animals are at least free from:
Hatred and delusion.
They may become greedy/lustful when they are hungry that is true, but often they are not, and perhaps they are never at all, because it's not a mental state for them of thinking.
“When lust, hate, and delusion are abandoned, a man does not choose for his own affliction or
for others’ affliction or for the affliction of both. In that way there comes to be Nibbāna here and
now, without delay, inviting inspection, onward-leading, and experienceable by the wise.
Why did the Buddha say it's a lower realm? well maybe because they can't consciously choose to transcend, are at the whim of their circumstances.
Or because it is a metaphor for the lower human state of craving without conscious wisdom,
but I am not sure if he said that that means they aren't enlightened.
Anyway, they often show great peace and happiness in my view, often no craving, just being.