I found this explanation from a N.I.V BIBLE COMMENTARY:
John 10:16
OTHER SHEEP = It is commonly understood that Jesus was referring to the future Gentile believers as His "other sheep." He was telling His Jewish disciples that His flock would include believers from all nationalities. But Jehovah's Witnesses attach a different meaning to this text. They contrast the "other sheep" here with the "little flock" mentioned in Luke 12:32. JWs say the "little flock" are the 144,000 spirit-anointed believers who make up the body of Christ and who will go to heaven, while the "other sheep" include all other believers who receive everlasting life here on earth. JWs teach that the opportunity to become part of the "little flock" ended back in the year 1935, so more than 99 percent of JWs today consider themselves in the "other sheep" class. To refute the doctrine that Christians are divided into heavenly and earthly classes, see Revelation 7:4 (about the "little flock" of the 144,000), and Revelation 7:9 (about the "great crowd" of "other sheep"). In addition, JWs teach that all pre-Christian believers are in the "other sheep" class, thus Abraham, Isaac, Jacob, the prophets, and so on, do not go to heaven. The best response is to read Hebrews 11:1-40 with a special emphasis on Hebrews 11:13, 16 showing they desired a "heavenly" country and a "city." Hebrews 12:22 indicates this is the heavenly Jerusalem. Paul contrasts the enemies of Christ as those who have their "minds on earthly things" while the Christian has his "citizenship in heaven" (Philippians 3:19, 20). Also see related JW texts in Psalm 37:9, 11; Psalm 37:29; Psalm 115:16; Luke 23:43; and Revelation 7:9.