El muslo está jugoso.

Spanish often uses estar for a state or condition, especially when talking about how food turns out after cooking.

So está jugoso means the thigh is juicy in the sense of its current texture or result.
Using es jugoso would sound more like describing an inherent or defining characteristic, which is much less natural here.

With food, Spanish commonly says things like:

• La carne está tierna = The meat is tender
• El pescado está seco = The fish is dry
• El pollo está delicioso = The chicken is delicious

So in this sentence, está is the normal choice.

Muslo literally means thigh. It can refer to:

• a person’s thigh
• an animal’s thigh
• a piece of meat, especially chicken

In a food context, el muslo very often means the thigh piece of the chicken or another bird. So whether it means a human thigh or a food item depends on context.

Because muslo is a masculine noun in Spanish, so it takes the masculine singular article el.

• el muslo
• un muslo
• este muslo

This is something you mostly just have to learn with the noun. The ending -o is a strong clue that it is masculine, though not every noun follows that pattern.

Because adjectives in Spanish usually agree with the noun they describe in gender and number.

Here, muslo is:

• masculine
• singular

So the adjective must also be:

• masculine
• singular

That gives jugoso.

Other forms would be:

• el muslo jugoso = masculine singular
• la pechuga jugosa = feminine singular
• los muslos jugosos = masculine plural
• las piezas jugosas = feminine plural

Grammatically, yes, but in most normal food contexts it is less natural.

Es jugoso would suggest something like:

• it is characteristically juicy
• it is, by nature, a juicy cut

That kind of wording can work in a more general or descriptive statement, but if you are talking about the food in front of you or how it came out after cooking, está jugoso is the usual Spanish choice.

So:

• El muslo está jugoso = This thigh is juicy / The thigh turned out juicy
• El muslo es jugoso = The thigh is a juicy cut / Thigh meat is juicy

No. Jugoso can be used literally and figuratively.

Literal use:

• La carne está jugosa = The meat is juicy

Figurative use:

• un contrato jugoso = a lucrative contract
• una oferta jugosa = a very attractive offer
• un detalle jugoso = a juicy detail / interesting gossip-like detail

In your sentence, though, it is clearly the literal food meaning.

A rough Spain Spanish pronunciation is:

el MOOS-lo es-TAH hoo-GO-so

A few helpful points:

• muslo: the s and l are both pronounced, though the cluster may feel awkward at first
• está: the stress is on the second syllable because of the accent mark
• j in jugoso is a strong throaty sound in Spain, like the ch in Scottish loch or German Bach
• g in go is a normal hard g

A more detailed approximation: el MUS-lo es-TA ju-GO-so

The accent mark shows that the stress falls on the last syllable: es-TÁ.

It also helps distinguish está from esta:

• está = is
• esta = this (feminine)

So the accent is important for both pronunciation and meaning.

Sometimes, but not always.

In El muslo está jugoso, the article sounds natural because you are talking about the thigh as a specific item or portion.

You might omit the article in some contexts, especially in short menu-style or note-like language, but in a full normal sentence, el is what you would usually expect.

Compare:

• El muslo está jugoso = The thigh is juicy
• Muslo jugoso = juicy thigh / thigh, juicy style / menu-style phrase, not a full equivalent in normal speech

Yes, it is natural and correct in Spain Spanish.

A speaker in Spain might also say similar things like:

• El muslo está muy jugoso = The thigh is very juicy
• Está en su punto y jugoso = It’s perfectly cooked and juicy
• La carne del muslo está jugosa = The thigh meat is juicy

So your sentence sounds normal and idiomatic.

The plural would be:

Los muslos están jugosos.

Here is what changes:

• el → los
• muslo → muslos
• está → están
• jugoso → jugosos

Everything agrees in number, which is very common in Spanish grammar.