Mahāmāyūrī Vidyārājñī also known as "[Great] Peacock Wisdom Queen" (Literal Translation of the Sanskrit)  is a [peaceful looking] wrathful deity in Mahayana. She is known as Kujaku Myo-o in Japanese.  She is generally associated with the removal of poisons. 

Mahamayuri Devi belongs to the class of deities called "Vidyārājas" (Wisdom Kings). She is one of the Pañcarakṣā Devi's [Five protection Deities] in the Nepalese Tradition. The others being "Mahāpratisarā", "Mahāsāhasra-pramardinī", "Mahāśītavatī", & "Mahāmantrānusāriṇī"

From the Iconography of Nepalese Buddhism :

The Mahāmayūrī vidyā seems to have appeared originally in the  "Bhaiṣajya Vastu" of the "Sarvāstivāda Vinaya" , from, which other derivations must have occured.

From Bulletin of Tibetology  - A Dharani Mantra of Vinayavastu :

(The paper summarizes the presence of this Vidya in the Sarvastiva Vinaya, as an early indication of Tantra in Pre-Schismatic Buddhism)

[...] It is interesting to note that 'Mahamayuri-Mantra' had been prescribed by Sakyaputra Gautama, the Buddha, himself when a monk was not cured in spite of the treatment of a Vaidya from his snake-bit. The account is mentioned in the Bhaisajya-vastu (T. Sman gyi gzhi) of the Mulasarvastivada Vinaya vastu ('Dul ba gzhi: Bka' - 'gyur. Nge. Vol. Peking Edn). As usual the method of narrating an account in the vinaya-texts is observed here. A monk named Sari (Skt. Svati) had a snake-bite.

In this connection a legend of the Peacock-king named suvarna-prabhasa depicts the efficacy of the Mahamayuri-Vidya who had bee conversant in the Vidya. He was in the right side of the Himalaya mountain when he was caught hold in a net of an enemy at the midnight after allure in the company of the peahens around him. He however regained his memory and chanted the Vidya. Thereafter, he could run away. The net was broken off.

[...] It becomes evident that the Vidya in Sanskrit had been prevalent in India [...]

In course of time the Mahamayuri Vidya became prominent for its power to sope snakes biting and it was called Vidyarajni. (Queen of the secret sciences. The Vidya was included in the list of the five protecting Dharanis (Parncaraksa) [...]

The Vidya of Mahamayuri

The Vidya of Mahamayuri as seen in the Bhaisajyavastu, from the e-text of Goettingen State and University Library.

There are two different versions of the Vidya. The Buddha says the first version, to cure the Bhikshu Svati, and the second version as the Peacock King to free himself from the net.

First Version of the Vidya:

namo buddhāya namo dharmāya namaḥ saṃghāya 

tadyathā amale vimale nirmale maṃgale hiraṇye hiraṇyagarbhe bhadre subhadre samantabhadre śrībhadre sarvārthasādhani paramārthasādhani sarvānarthapraśamani sarvamaṅgalasādhani manase mānase mahāmānase acyute adbhute atyadbhute mukte mocani mokṣaṇi araje viraje amṛte amare amaraṇi brahme brahmasvare pūrṇe pūrṇamanorathe mukte jīvati rakṣa svātiṃ sarvopadravabhayarogebhyaḥ svāhā 

Second Version of the Vidya: 

[ namo buddhāya namo dharmāya namaḥ saṃghāya ]

tadyathā amale vimale nirmale maṅgalye hiraṇye hiraṇyagarbhe bhadre subhadre samantabhadre śrībhadre sarvārthasādhani paramārthasādhani sarvānarthapraśamani sarvamaṅgalyasādhani manasi mānasi mahāmānasi acyute adbhute atyadbhute mukte mocani mokṣaṇi araje viraje amare amṛte amaraṇi brahme brahmasvare pūrṇe pūrṇamanorathe vimukte jīvati rakṣa māṃ sarvopadravebhyaḥ svāhā

Apart from the variation such as Mangale/Mangalye Manse/Manasi etc [which I think is mostly due to manuscript variations ] between the two versions, note the Italicized portions. In the first version, when the Buddha proclaims the vidya to save the Bhikshu Svati he says, "rakṣa svātiṃ" (Protect Svati), and in the second version, as the Peacock Suvarnaprabhasa, when remembering the dharani to help himself out,  the buddha says "rakṣa māṃ" (Protect Me).

Similar to that noted by Jayarava here, this could be an example of a "Template-style-dharani" too - The Pattern to be matched is  Raksha <<object in accusative case>>. 

Notes:

1. The e-text has "rakṣā svātiṃ" which must be a typo. The BoT has "rakṣa svātiṃ", which seems to be correct  "Protect<<imperative>> Me<<accusative>>", and matches more with the second version of the dharani. I have followed the later.