What are "Chayos"?
Radak citing Targum Yonasan: They are creations.
Malbim: They are one of the four classes of angels that surround the Kisei. At a time of Galus, Hashem normally conducts the world via the Tzeva and Ma'arachah 1 - Ofanim and Galgalim (spheres on which the Heavenly bodies are fixed). When He wants to conduct based on His will, via hidden miracles, he moves [the Ma'arachah] via the Chayos, who are above the Ofanim and move them. This is like one who rides in a chariot - he conducts it via the animals that transport it. The Chayos are below the Serafim that Yeshayah saw; they do Hashem's will without an intermediary. Therefore, Yechezkel saw them as Chayos, which are below the level of man.
Malbim: This conduct is fixed at a time of Galus, when the nations' Sarim rule as the Ma'arachah obligates. Hashem power is dressed in them, to tilt their conduct as He desires. The rule of the four kingdoms (that will subjugate Yisrael - Bavel, Pras and Madai, Yavan and Edom) began then. This will be until the Kisei will return to its place, and Hashem's name and Kisei will be complete, to conduct via the Serafim. Each kingdom suckles from one of the four Chayos under the Merkavah. Each overpowers in one of the four directions. The Sar influence on his nation as long as the star of his success shines, until it sinks in the pit and another Ofan will rise in place of it.
Why does it say that they have a human form? They have the form also of an ox, lion and Nesher!
Rashi: The human form is the 'father' of all of them, therefore he uses it to praise the Merkavah. It was the face of Yakov Avinu.
Malbim: An animal's form is unlike man's. It goes on four legs, it faces down, and it has no Sechel. The Chayos' bodies were like man - they stand on two legs and have two hands, and they are full of Chachmah and Tevunah (to fathom Hashem, and praise Him and honor His name), like man. They resembled animals only in [three of] their faces.
Why did Yechezkel describe his vision in much greater detail than Yeshayah (6:1)?
Radak (Yeshayah 6:1, citing Chagigah 13b): Yechezkel was like a villager who saw the king. Yeshayah was like a city resident who saw the king 1 . I.e. Yeshayah (or his generation) was at a higher level of perfection than Yechezkel (or his generation), so what he saw was not such a Chidush to him.
Malbim: The Merkavah that Yechezkel saw was in Olam ha'Yetzirah, the world of angels. It was below the Merkavah that Yeshayah saw, in Olam ha'Kisei. Yeshayah saw the Shechinah in its place, when Yisrael were on their land; the conduct was from Hashem Himself. "Hashem ba'Shamayim Hechin Kis'o u'Malchuso ba'Kol Mashalah" (Tehilim 103:19). Therefore, he saw Hashem sitting on an elevated throne. Also conduct of the Ma'arachah was from Him, without an intermediary. "Serafim Omedim mi'Ma'al Lo" (Yeshayah 6:2) - at the height of Olam ha'Kisei, above Olam ha'Chayos that Yechezkel saw. Yeshayah saw like a king in his house, and his officers are in front of him. Yechezkel saw in Golah, in Chutz la'Aretz. Then Hashem does not conduct miraculously, rather, according to nature and Ma'arachah (Ofanim); He moves them. The Kuzari (Ma'amar Chamishi) says that Nevi'im perceive Hashem based on His action at the time. Daniel (7:9) saw Him judging. Yechezkel saw Him riding in a chariot, when He was traveling to Golah, like kings ride in a chariot led via four Chayos (animals).
Malbim: The city resident sees the king in his palace on his throne, and his servants are in front of him. The villager sees him when he travels in a chariot.
