What's the meaning of 'relies on senses other than sight' in the Mirror Image spell?

Question

The mirror image spell description says

A creature is unaffected by this spell if it can't see, if it relies on senses other than sight, such as blindsight, or if it can perceive illusions as false, as with truesight.

Most creature has senses other than sight, but some has (far) better sense that works together with sight.

1. A Wolf has Keen Hearing and Smell. Does this mean wolf can bypass mirror image effect? The wolf does not rely on hearing or smell, but has 'heightened sense' on that.

2. Dragons have Blindsight 10 ft, but they can still see. Does this mean they still rely on sight and affected by mirror image even in 10 ft?

3. A Shambling Mound has Blindsight and blind beyond 60 ft. It seems to me that it does not have normal sight. It does not rely on sight and would not be affected by mirror image.

What does the rule say about this?

Answer 1

When not using vision to target the recipient of mirror image, the effects of the spell do not apply.

tl;dr Either attack at disadvantage not using sight or have blindsight

Creatures with blindsight are unaffected by mirror image

Creatures with blindsight also come with the condition immunity to being blinded. This tells us that they do not depend on sight. They are completely unaffected by mirror image.

Tremorsense

Tremorsense itself is unaffected by mirror image, but since tremorsense only gives the position of a foe, relying on it alone still incurs disadvantage as the foe cannot be seen. Similar to knowing the position of an invisible foe.

Take our friend the bulette for example. Tremorsense is useful for finding prey hiding on and under the ground, but it still has darkvision and can still be blinded.

Close your eyes

Closing ones eyes to rely on other senses before attacking would avoid the effect of mirror image but also put the attacker at least at disadvantage. The attacker is also further at risk of attacking an empty space as the mirror image foe might have snuck away while their eyes were closed.

Answer 2

Keen Smell and Hearing

Keen only gives advantage on perception checks with those senses, so as written it isn't enough. If a wolf closed its eyes, it'd be attacking at disadvantage; thus we know it relies on its sight. That said, I am personally tempted to rule that with perception check vs the spell the DC if it came up at my table.

Keen Hearing and Smell. The wolf has advantage on Wisdom (Perception) checks that rely on hearing or smell.

Blindsight

The mirror image spell specifically lays out that blindsight is one such method of not relying on sight:

if it relies on senses other than sight, such as Blindsight

You ask:

Dragons have Blindsight 10 ft, but they can still see. Does this mean they still rely on sight and affected by mirror image even in 10 ft?

So Blindsight says:

A monster with blindsight can perceive its surroundings without relying on sight, within a specific radius.

So creatures with blindsight are unaffected by mirror image while within the specific radius of their blindsight.