I believe they are angels.
I will supply five lines of evidence for this conclusion:
1. Evidence that pneuma in 1 Peter 3:18-20 is referring to fallen angels.
2. Evidence that these are spirits that sinned in the time of Noah.
3. Evidence that their sin was the sin of fornication.
4. Evidence that the "Sons of God" in the OT refers to angels.
5. Evidence that this was the common interpretation by the Jews of the NT days.
1. Evidence that pneuma in 1 Peter 3:18-20 is referring to fallen angels.
1 Pet 3:18-20 has a very interesting reference to "spirits" who sinned in the time of Noah.
In Heb 1:7, 14 pneuma is used to designate angels.
In Matt. 8:16 pneuma is used to designate demons.
Kenneth Wuest the Greek scholar in his commentary called "First Peter in the Greek New Testament" (pp98-106 covers the entire discussion) speaking about 1 Peter 3:19-20 writes, " 'Who are these spirits?' They cannot be human beings, for a careful study of the Greek word "spirit" pneuma in the Greek New Testament will reveal the fact that in no place is the word used as a designation of a human being when the writer has in mind a human being considered as a free moral agent in a distinctive category or class of created beings. The word is used where a human being is said to have a spirit, referring to that part of a person which enables him to have God-consciousness and which constitutes him a religious being (Luke 1:47). It is used of the disembodied state of human beings in the phrase, "the spirits of just men made perfect" (Heb. 12:23). But in this latter phrase, it does not designate the human being as a class, distinct in the order of created beings, but speaks of the disembodied state of that human being."
So the spirits of 1 Peter 3 must be fallen angels. 2. Evidence that these are spirits that sinned in the time of Noah.
1 Peter 3:20 says they sinned at the time of Noah. It appears to be talking about the same group of spirits that Peter mentioned in 1 Peter 3:20 because in both contexts Noah is discussed.
2 Peter 2:4-5 refers to Tartarus. There are just two prisons in the unseen world where spirits are kept, Tartarus and Abusson, the bottomless pit. Of Tartarus Wuest (page 102) says, "The Greek poet Homer uses the term 'Hades' as the place for dead men and 'Tartarus,' a murky abyss beneath Hades, for fallen immortals. Peter uses the word in agreement with the Book of Enoch and with Greek mythology, because he is speaking of fallen angels, not men."
3. Evidence that their sin was the sin of fornication.
Jude 6-7 in a parallel discussion to 2 Peter 2:4-6 tells us that their sin was fornication. These angels who did not keep their own domain "arche" (or first order or office and dignity) or the exalted position of the angels in heaven, "in contradistinction to the lower place occupied by the earth dwellers". (Wuest page 102) i.e. they left their position which was higher than man and lowered themselves to act like men. "He has kept in eternal bonds under darkness for the day of judgment (reminds us of the 'Pits of darkness, reserved for judgment" of 2 Peter). "Just as Sodom and Gomorrah and the cities around them, since they even as these (as these angels). Wuest says, "The words 'even as' are the translation of an adverb which means literally 'like as, in the same manner as." The sin of these angels was of the same character as that committed by the nhabitants of the cities mentioned in (Jude) 7."
He goes on to say, "This means that the comparison is between the angels of verse 6 and the cities of verse 7. This interpretation must be conclusive, all opinions to the contrary notwithstanding, for it is based upon the rules of Greek grammar. The sin in both cases is said to be fornication. We have the definite statement of Scripture therefore that the sin of the fallen angels was fornication." (Wuest pg. 102-103)
Matt 12:43-45 shows us that demons do take up residence in physical bodies. This may infer that at one time they had physical bodies and, being deprived of them though some judgment of God, they try to satisfy their desire for a physical existence by indwelling physical bodies.
4. Evidence that the "Sons of God" in the OT refers to angels.
Gen 6:1-4 speaks of the "Sons of God" which Job (who wrote before Moses) had used clearly of angels Job 1:6, 2:1, and 38:7.
5. Evidence that this was the common interpretation by the Jews of the NT days. This interpretation is not contradicted by Peter and Jude but instead was reinforced by them in using the termonology (Tartarus) and similiar descriptions of the events in Noah's day used by other Jewish writers.
a. The apocraphal Book of Enoch which Jude apparently quotes from (compare Jude 14-15 with The Book of Enoch chapter 1:9 " And behold! He cometh with ten thousands of His holy ones To execute judgement upon all, And to destroy all the ungodly: And to convict all flesh Of all the works of their ungodliness which they have ungodly committed, And of all the hard things which ungodly sinners have spoken against Him.")
The Book of Encoh 6-23 also has an account of the fallen angels of Gen 6 (a small sample is shown here ([Chapter 7]
1 And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go in unto them and to defile themselves with them, and they taught them charms 2 and enchantments, and the cutting of roots, and made them acquainted with plants. And they 3 became pregnant, and they bare great giants, whose height was three thousand ells: Who consumed 4 all the acquisitions of men. And when men could no longer sustain them, the giants turned against 5 them and devoured mankind. And they began to sin against birds, and beasts, and reptiles, and 6 fish, and to devour one another's flesh, and drink the blood. Then the earth laid accusation against the lawless ones.)
The Book of Enoch also mentions Tartarus, (The Book of Enoch 20:3 1,2 And these are the names of the holy angels who watch. Uriel, one of the holy angels, who is 3 over the world and over Tartarus. Raphael, one of the holy angels, who is over the spirits of men.)
b. Wuest (page 102 says, "Josephus, Jewish historian fot he first century, speaks of the sons of God of Genesis 6 as angles. and in such a way as to indicate that thet was the commly accepted interpretation in his day."
c. Wuest (page 103) also says, "Justin Martyr speaks of the angels who violated the taxin of women by intercourse with them."
Why did Satan want his angels to have relations with women? Apparently to create a mixed race, part human, part angel, which would have interfered with the plan of God to send a second Adam (Christ) born of the seed of woman. God had to exterminate almost all of those alive at the time to assure that Satan's infection of the human race with his demons did not prevent the Messiah from coming.
The proclamation in 1 Peter 3 may have been a declaration to the angels that sinned and were kept in darkness, (both spiritual and informational-unaware that Christ had come, foiling their plan) that God had defeated the scheme of Satan, and therefore it was a proclamation of victory.