I believe that the riders in chapter 6 represent what the Anti-Christ will do during the first 3 1/2 years of the tribulation, so it's most likely, in my opinion, that the riders (besides the fourth one), are the Antichrist at different points in the beginning of the trib. As for the beast from the bottomless pit, I think that's quite possibly Satan, and that Satan will be controlling the Anti-Christ.
In Revelation 11:7, it says that the beast from the bottomless pit will make war against the two witnesses, overcome them, and kill them. Daniel also says that the little horn which he saw (the Anti-Christ) will make war with the saints (Israel) and prevail against them. By the way, we know that these saints are not the church because in Matthew 16:18, Christ told Peter that the gates of Hades would not prevail against the church. Anyway, the beast in the bottomless pit could be referring to the Anti-Christ, though it's hard to connect the fact that a man comes from there. I believe it's talking about Satan, and Satan will also be making war with Israel and kill many of them, but He will be doing it by using the Anti-Christ. Also Revelation speaks of Satan spewing a flood (not exactly sure if this is literal or symbolic for an army) after the woman (Israel), to wipe them out, though that attack is not successful. Another reason why I think the Anti-Christ will be controlled by Satan is that they are both described as having seven heads and ten horns.
So I think that the Bible refers to three different beasts/persons:
beast from the bottomles pit - Satan, beast from the sea - Anti-Christ, beast from the earth - False Prophet. What an unholy trinity! Jesus Christ was the fleshly incarnate of God. Satan cannot create human flesh, or an incarnate of himself. So what does he do instead? He possess' someone, the Anti-Christ. Christ took it upon Himself to come into the flesh and save our souls, and I think that the there will be a man one day who takes it upon himself to blasphemy the Creator of the universe. He will sell his soul to the devil, and will pay dearly for it.