Spare is an animatronic in Five Nights at Freddy's 5, he is a spare Freddy Fazbear suit that needs to be cleaned and is kept in the storeroom of the museum.
Spare has the same design as Freddy in the original Five Nights at Freddy's, but is stained red and purple. He has sustained some damage, but little compared to the other museum attractions. Closer inspection shows that his eyes are very physically similar to Springtrap's eyes, and like Springtrap Spare has small pieces of human flesh exposed.
Spare becomes active from night 3 onward. He starts in the storeroom and will move directly to the Staff Hall, then to the hall outside your office. When he is in the hall there will be a noise (the noise in question is in fact a random phone call from FNAF 1, played backward and very slowly), making it easy to tell when he is there. Once Spare gets into your office, you have about one second to put on your Puppet Mask before he will jumpscare you.
The phone call on Night 3 claims that Spare was discovered in the backroom of Freddy Fazbear's Pizza, and that he s in better condition than the Freddy suit already on display, but that he needs to be cleaned before he can be displayed. One of the game's death minigames elaborates on this. It has you play as The Puppet, following a semi-random path through the FNAF 1 pizzeria, but the trail always ends in the office, where slumped body is present, and when you walk up to the phone, it shows the words "Message Recorded", suggesting that the dead figure is the Phone Guy, after his death on night 4. Once the phone is walked up to, Spare's jumpscare will play, and you will be brought back to the main menu. This suggests that the human body is spare is that of the Phone Guy.