Cuando hace calor, prefiero llevar una coleta y no el pelo suelto.

In Spanish, weather and temperature are often expressed with hacer:

• Hace calor = It’s hot
• Hace frío = It’s cold
• Hace buen tiempo = The weather is nice

So hacer is the normal verb here, even though it does not translate word-for-word into English. Es calor is not how Spanish expresses this idea.

Here, cuando hace calor can be understood as when it’s hot or whenever it’s hot.

Because the sentence describes a general habit or preference, English often naturally hears it as whenever it’s hot:

• Cuando hace calor, prefiero...
  = When/Whenever it’s hot, I prefer...

So both ideas are present, but in this context it is more like a repeated situation, not one specific moment.

After preferir, Spanish often uses an infinitive when the same person does both actions:

• Prefiero llevar una coleta
  = I prefer to wear my hair in a ponytail

Structure:

• prefiero = I prefer
• llevar = infinitive, to wear / to have on / to keep

This is very similar to English prefer to + verb.

Other examples:

• Prefiero comer en casa. = I prefer to eat at home.
• Prefiero esperar. = I prefer to wait.

Yes, llevar often means to carry or to take, but it also very commonly means to wear.

In Spanish, you can use llevar for:

• clothes: Lleva una camisa azul.
• accessories: Lleva gafas.
• hairstyles or beard styles: Lleva coleta., Lleva barba.

So llevar una coleta means to wear one’s hair in a ponytail.

This is a very natural use of llevar.

In Spain, una coleta usually means a ponytail.

That said, vocabulary can vary by region. In some other Spanish-speaking places, people may more often say:

• cola de caballo = ponytail

For Spain Spanish, coleta is very normal and natural.

Because the sentence is talking about two different kinds of expressions:

1. una coleta
   This is a countable noun phrase: a ponytail.
   So it takes an indefinite article: una.

2. el pelo suelto
   Pelo is usually treated as a general/body-part noun here: the hair worn loose.
   Spanish often uses the definite article with body parts where English uses my/your.

So:

• llevar una coleta = to wear a ponytail
• llevar el pelo suelto = to wear one’s hair loose

This is a very common Spanish pattern. With body parts, clothing, and personal appearance, Spanish often uses the definite article instead of a possessive when ownership is already obvious.

So Spanish prefers:

• Me duele la cabeza. = My head hurts.
• Lleva las manos limpias. = He/She has clean hands.
• Llevo el pelo corto. = I wear my hair short.

In your sentence, it is clear that the speaker is talking about their own hair, so el pelo sounds natural.
Using mi pelo is possible in some contexts, but here it would sound less idiomatic.

Here suelto means loose, meaning not tied up.

So:

• el pelo suelto = hair worn loose / hair down

It is the opposite of having the hair tied back in a ponytail, bun, braid, etc.

You may also hear:

• llevar el pelo suelto = to wear your hair down
• dejarse el pelo suelto = to leave your hair down

Because suelto agrees with pelo, and pelo is masculine singular.

• el pelo → masculine singular
• therefore → suelto

If the noun were feminine, the adjective would change:

• la melena suelta

And if plural:

• los pelos sueltos
• las trenzas sueltas

So this is standard adjective agreement.

Spanish often avoids repeating words that are already understood.

The full idea would be something like:

• Cuando hace calor, prefiero llevar una coleta y no llevar el pelo suelto.

But repeating llevar is unnecessary, so Spanish leaves it out:

• ...prefiero llevar una coleta y no el pelo suelto.

This kind of ellipsis is very common and natural. English does it too:

• I prefer a ponytail and not loose hair

even though the full version would be

• I prefer wearing a ponytail and not wearing my hair loose.

Yes, pero no el pelo suelto is possible, and it makes the contrast a bit more explicit.

Compare:

• ...y no el pelo suelto = and not loose hair
• ...pero no el pelo suelto = but not loose hair

In this sentence, y no sounds perfectly natural because it links two alternatives in a simple contrast.
Pero no would also work, but it may sound slightly more emphatic.

Because Spanish usually omits subject pronouns when the verb form already shows who the subject is.

• prefiero already means I prefer

So:

• Prefiero llevar una coleta... = I prefer to wear a ponytail...

You could say yo prefiero if you want emphasis or contrast:

• Yo prefiero llevar una coleta, pero mi hermana no.

But in a neutral sentence, leaving out yo is more natural.

Yes, it is standard and helpful here.

Cuando hace calor is an introductory subordinate clause, so a comma before the main clause is normal:

• Cuando hace calor, prefiero llevar una coleta...

This is similar to English:

• When it’s hot, I prefer...

If the order were reversed, there would usually be no comma:

• Prefiero llevar una coleta cuando hace calor.

Yes. A few natural alternatives in Spain Spanish are:

• Cuando hace calor, prefiero llevar coleta y no el pelo suelto.
• Cuando hace calor, prefiero llevar el pelo recogido y no suelto.
• Cuando hace calor, prefiero llevar el pelo en una coleta.

Notes:

• llevar coleta without una is also common in everyday speech.
• el pelo recogido means hair tied up / hair up
• el pelo suelto means hair down / loose

Your original sentence is completely natural, but these are useful variants you may hear.