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I no longer agree with my answer but instead with the one presented above by Slagmoth, but leave it here for you to read and make your own decision:

My argument is that yes casting a Dispel Magic on one of the creatures ends the whole spell.

Conjure Animals:

3rd-level conjuration

 

You summon fey spirits that take the form of beasts and appear in unoccupied spaces that you can see within range.

 

Each beast is also considered fey, and it disappears when it drops to 0 hit points or when the spell ends.

Conjure animals is a 3rd level spell that summons a number of fey spirits in the form of beasts and when it ends all the animals disappear together. Therefore it is a single magical effect, no matter how far apart the animals get, or how far from the caster they get. For instance the spell ends when the caster loses concentration and all the animals disappear as one. It is a single magical effect. This is a key point.

Dispel magic:

Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends.

The key point is that, as with any spell, if any part of a target is in range of a Dispel Magic you can target it.

Therefore if you can cast a Dispel Magic on any one of the fey spirits that are there because of the conjuration spell, you have targeted the magical effect, the spell ends and all the creatures disappear as one.

Targeting the caster does not end the spell as they are not part of the magical effect nor do they have the spell "on" them, despite the need for their concentration.

I no longer agree with my answer but instead with the one presented above by Slagmoth, but leave it here for you to read and make your own decision:

My argument is that yes casting a Dispel Magic on one of the creatures ends the whole spell.

Conjure Animals:

3rd-level conjuration

You summon fey spirits that take the form of beasts and appear in unoccupied spaces that you can see within range.

Each beast is also considered fey, and it disappears when it drops to 0 hit points or when the spell ends.

Conjure animals is a 3rd level spell that summons a number of fey spirits in the form of beasts and when it ends all the animals disappear together. Therefore it is a single magical effect, no matter how far apart the animals get, or how far from the caster they get. For instance the spell ends when the caster loses concentration and all the animals disappear as one. It is a single magical effect. This is a key point.

Dispel magic:

Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends.

The key point is that, as with any spell, if any part of a target is in range of a Dispel Magic you can target it.

Therefore if you can cast a Dispel Magic on any one of the fey spirits that are there because of the conjuration spell, you have targeted the magical effect, the spell ends and all the creatures disappear as one.

Targeting the caster does not end the spell as they are not part of the magical effect nor do they have the spell "on" them, despite the need for their concentration.

My argument is that yes casting a Dispel Magic on one of the creatures ends the whole spell.

Conjure Animals:

3rd-level conjuration

You summon fey spirits that take the form of beasts and appear in unoccupied spaces that you can see within range.

Each beast is also considered fey, and it disappears when it drops to 0 hit points or when the spell ends.

Conjure animals is a 3rd level spell that summons a number of fey spirits in the form of beasts and when it ends all the animals disappear together. Therefore it is a single magical effect, no matter how far apart the animals get, or how far from the caster they get. For instance the spell ends when the caster loses concentration and all the animals disappear as one. It is a single magical effect. This is a key point.

Dispel magic:

Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends.

The key point is that, as with any spell, if any part of a target is in range of a Dispel Magic you can target it.

Therefore if you can cast a Dispel Magic on any one of the fey spirits that are there because of the conjuration spell, you have targeted the magical effect, the spell ends and all the creatures disappear as one.

Targeting the caster does not end the spell as they are not part of the magical effect nor do they have the spell "on" them, despite the need for their concentration.

I no longer agree with my answer but instead with the one presented above by Slagmoth, but leave it here for you to read and make your own decision:

My argument is that yes casting a Dispel Magic on one of the creatures ends the whole spell.

Conjure Animals:

3rd-level conjuration

You summon fey spirits that take the form of beasts and appear in unoccupied spaces that you can see within range.

Each beast is also considered fey, and it disappears when it drops to 0 hit points or when the spell ends.

Conjure animals is a 3rd level spell that summons a number of fey spirits in the form of beasts and when it ends all the animals disappear together. Therefore it is a single magical effect, no matter how far apart the animals get, or how far from the caster they get. For instance the spell ends when the caster loses concentration and all the animals disappear as one. It is a single magical effect. This is a key point.

Dispel magic:

Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends.

The key point is that, as with any spell, if any part of a target is in range of a Dispel Magic you can target it.

Therefore if you can cast a Dispel Magic on any one of the fey spirits that are there because of the conjuration spell, you have targeted the magical effect, the spell ends and all the creatures disappear as one.

Targeting the caster does not end the spell as they are not part of the magical effect nor do they have the spell "on" them, despite the need for their concentration.

The answer by Slagmoth carries the day for me and I withdraw the answer I set out below. I'll leave it here so people can read it if they want, but Slagmoth's answer is based on a cannonical RAW source and is simple and transferable.

My argument is that yes casting a Dispel Magic on one of the creatures ends the whole spell.

Conjure Animals:

3rd-level conjuration

You summon fey spirits that take the form of beasts and appear in unoccupied spaces that you can see within range.

Each beast is also considered fey, and it disappears when it drops to 0 hit points or when the spell ends.

Conjure animals is a 3rd level spell that summons a number of fey spirits in the form of beasts and when it ends all the animals disappear together. Therefore it is a single magical effect, no matter how far apart the animals get, or how far from the caster they get. For instance the spell ends when the caster loses concentration and all the animals disappear as one. It is a single magical effect. This is a key point.

Dispel magic:

Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends.

The key point is that, as with any spell, if any part of a target is in range of a Dispel Magic you can target it.

Therefore if you can cast a Dispel Magic on any one of the fey spirits that are there because of the conjuration spell, you have targeted the magical effect, the spell ends and all the creatures disappear as one.

Targeting the caster does not end the spell as they are not part of the magical effect nor do they have the spell "on" them, despite the need for their concentration.

My argument is that yes casting a Dispel Magic on one of the creatures ends the whole spell.

Conjure Animals:

3rd-level conjuration

You summon fey spirits that take the form of beasts and appear in unoccupied spaces that you can see within range.

Each beast is also considered fey, and it disappears when it drops to 0 hit points or when the spell ends.

Conjure animals is a 3rd level spell that summons a number of fey spirits in the form of beasts and when it ends all the animals disappear together. Therefore it is a single magical effect, no matter how far apart the animals get, or how far from the caster they get. For instance the spell ends when the caster loses concentration and all the animals disappear as one. It is a single magical effect. This is a key point.

Dispel magic:

Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends.

The key point is that, as with any spell, if any part of a target is in range of a Dispel Magic you can target it.

Therefore if you can cast a Dispel Magic on any one of the fey spirits that are there because of the conjuration spell, you have targeted the magical effect, the spell ends and all the creatures disappear as one.

Targeting the caster does not end the spell as they are not part of the magical effect nor do they have the spell "on" them, despite the need for their concentration.