According to Sabha Parva - SECTION III-Sabha Kriya Parva Of Mahabharata , Kinkaras are the the race of fierce, huge-bodied Rakshasas with coppery eyes and arrowy ears.
In Mahabharata they are said to be guards and protectors of Maya-Sabha ,which Maya Danava made for Pandavas after the Kahndav Dahana. They were said to be carrying this hall anywhere from their strength. And their Number was 8000.
तां सम तत्र मयेनॊक्ता रक्षन्ति च वहन्ति च |
सभाम अष्टौ सहस्राणि
किंकरा नाम राक्षसाः || 25||
अन्तरिक्षचरा घॊरा महाकाया महाबलाः|
रक्ताक्षाः पिङ्गलाक्षाश च शुक्तिकर्णाः परहारिणः ||26||
Maya built was so wide, delightful, and refreshing, and composed of
such excellent materials, and furnished with such golden walls and
archways, and adorned with so many varied pictures, and was withal so
rich and well-built, that in beauty it far surpassed Sudharma of the
Dasarha race, or the mansion of Brahma himself. And eight thousand
Rakshasas called Kinkaras, fierce, huge-bodied and endued with great
strength, of red coppery eyes and arrowy ears, well-armed and capable
of ranging through the air, used to guard and protect that palace.
There is also a brief mentioning of Kinkara again in Adi Parva.
Mahabharata - Book 1: Adi Parva - SECTION CLXXVIII - Chaitraratha Parva - Tells us that a Rakshasa called Kinkara entered into King kalmashapada's Body on order of Rishi Vishwamitra.
Viswamitra, ascertaining the disposition of the king (and fearing that
the difference might be made up), ordered a Rakshasa to enter the body
of the king. And a Rakshasa of the name of Kinkara then entered the
monarch's body in obedience to Saktri's curse and Viswamitra's
command. p. 358
So it looks like they were race of Rakshasas in the command of Maya-Danava and were capable to fly in the air and to lift huge loads.