Is it known who first associated the Beast of Revelation with the Antichrist, and when?

It was John the Revelator–the same who authored The Revelation–but he doesn't say it in The Revelation.

"Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; [...] *Who is a liar but he that denieth that Jesus is the Christ?* He is antichrist, that denieth the Father and the Son." (1 John 2:18-22)

"And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world..." (1 John 4:3-6)

"For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an antichrist." (2 John 1:7)

John very strongly emphasized in his epistles that an antichrist is a deceiver and would lead people away from worshiping Christ.

And I beheld another beast coming up out of the earth [...] And he exerciseth all the power of the first beast before him [...] And he doeth great wonders [...] And deceiveth them that dwell on the earth [...] and cause that as many as would not worship the image of the beast should be killed... (Revelation 11-17)

John made it clear that any who would deceive the people and have them worship anyone but the Saviour was Antichrist. The second beast used great wonders and power to force the people of earth to worship the image of the beast or be killed, and caused all to receive the mark of the beast-a symbol of their worship.

This beast is textbook antichrist. The connection would not have required explanation at the time The Revelation was given to the church.

Justin Martyr (c. 100 – 165 AD, Dial. c. Trypho, cxx. 14, 15), who wrote before Irenaeus, mentions the Martyrdom of Isaiah, an earlier Christian writing with an apocalypse section (chapter 4). In it the author ties together the writings of Paul, Daniel, the Gospels and Revelation with the Belial/Beliar tradition to describe an evil antichrist type ruler. 1 John 2:18 says that christians were already expecting antichrist before John contrasted the idea by saying there were already many antichrists and a spirit. The church historian Eusebius (Theophania IV.35) confirms that the apostles were expecting an antichrist which Jesus spoke of (John 5:43) and it was the same one Paul spoke of (2 Thes 2:3).

1Jn 2:22 Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son.

4:6. And all that he hath desired he will do in the world: he will do and speak like the Beloved and he will say: "I am God and before me there has been none." 7. And all the people in the world will believe in him. 8. And they will sacrifice to him and they will serve him saying: "This is God and beside him there is no other."

You can't read the Ascension of Isaiah without noticing the allusions to Revelation, John's letters, the Gospels, Paul, Daniel and Belial.

St. Victorinus, an ecclesiastical writer and a Church Father who flourished about 270, and who suffered martyrdom probably in 303 under Diocletian, in his Commentary on the Apocalypse, identifies the Antichrist with one of the seven heads of the red dragon [cf. Rev 12:3].

From the Twelfth Chapter

3. And there appeared another sign in heaven; and behold a red dragon, having seven heads. Now, that he says that this dragon was of a red colour— that is, of a purple colour— the result of his work gave him such a colour. For from the beginning (as the Lord says) he was a murderer; and he has oppressed the whole of the human race, not so much by the obligation of death, as, moreover, by the various forms of destruction and fatal mischiefs. His seven heads were the seven kings of the Romans, of whom also is Antichrist, as we have said above.

Further reading

• Antichrist | New Advent. (Note: No unanimity as to who precisely is the Antichrist in the Apocalypse.)
• The Church Fathers on the Antichrist. (Please note what St. Justin Martyr and St. Irenaeus say.)
• The Antichrist | Catholic Answers.