From BiblePortal Wikipedia

Charles Buck Theological Dictionary [1]

Ancient heretics, who denied the duplicity of natures in Christ; thus denominated from Eutyches, the archimandrite or abbot of a monastery, at Constantinople, who began to propagate his opinion about A. D. 448. He did not, however, seem quite steady and consistent in his sentiments; for he appeared to allow of two natures, even before the union, which was apparently a consequence he drew from the principles of the Platonic philosophy, which supposes a pre-existence of souls: accordingly he believed that the soul of Jesus Christ had been united to the Divinity before the incarnation; but then he allowed no distinction of natures in Jesus Christ since his incarnation. This heresy was first condemned in a synod held at Constantinople, by Flavian, in 448; approved by the council of Ephesus, called convenitus latronum, in 449; and re-examined and fulminated in the general council of Chalcedon, in 451. The Eutychians were divided into several branches, as the Agnoetae, Theodosians, Severians, &c. &c. &C. Eutychians was also the name of a sect, half Arian and half Eunomian, which arose at Constantinople in the fourth century.

Watson's Biblical & Theological Dictionary [2]

a denomination which arose in the fifth century, and were so called from Eutyches, abbot of a certain convent of monks at Constantinople. The Nestorians having explained the two natures in Christ in such a manner as, in the opinion of many, to make them equivalent to two persons, which was an evident absurdity, Eutyches, to avoid this error, fell into the opposite extreme, and maintained that there was only one nature in Jesus Christ, the divine nature, which, according to him, had so entirely swallowed up the human, that the latter could not be distinguished. Hence it was inferred that according to this system our Lord had nothing of humanity but the appearance.