Where is the story of Lord Śiva beheading Brahmā who was in the form of a stag (deer) found?

Question

Prajāpati Brahmā in a desire to create more perfect and beautiful beings in the universe was obsessed with his beautiful daughter Rohiṇī. Sensing her father’s attraction and intention she assumed the form of a deer and fled. He kept chasing her in the form of a Stag. Lord Śiva sensing the incestuous intentions beheaded the stag, which became the constellation Mṛgaśiras (Deer-head).

Answer 1

Śiva (Rudra/ Bhūtavān) beheading Prajāpati who assumed the form of a male deer chasing his daughter in her form as a female deer (who later came to be known as the constellation Rohiṇī) is found in Aitareya Brāhmaṇa 3.3.9.

From the translation by Martin Haug:

Prajāpati thougnt of cohabiting with his own daughter, whom some call "Heaven," others "Dawn," (Uṣās). He transformed himself into a buck or a kind of deer (ṛṣya), whilst his daughter assumed the shape of a female deer (rohit). He approached her. The gods saw it (crying), "Prajāpati commits an act never done (before)." (In order to avert the evil consequences of this incestuous act) the gods inquired for some one who might destroy the evil consequences (of it). Among themselves they did not find any one who might do that (atone for Prajāpati's crime). They then put the most fearful bodies (for the gods have many bodies) of theirs in one. This aggregate of the most fearful bodies of the gods became a god, Bhūtavān, by name. For he who knows this name only, is born." The gods said to him, "Prajāpati has committed an act which he ought not to have committed. Pierce this (the incarnation of his evil deed)." So he did. He then said, "I will choose a boon from you." They said, "Choose." He then chose as his boon sovereignty over cattle." That is the reason that his name is paśumān), ie., having cattle. He who knows on this earth only this name (paśumān), becomes rich in cattle.He (Bhūtavān) attacked him (the incarnation of Prajāpati's evil deed) and pierced him (with an arrow). After having pierced him, he sprang up (and became a constellation). They call him mṛga, i.e., deer (stars in the Orion), and him who killed that being" (which sprang from Prajāpati's misdeeds), mṛga vyādha, i.e., hunter of the deer (name of star). The female deer Rohit (into which Prajāpati's daughter had been transformed) became (the constellation) Rohiṇī.

Answer 2

First of all, everything from your question is actually false. The true is:-

1. Lord Shiva beheading Brahma's fifth head

The Shiva, Skanda and Linga Puranas are clearly mentioned about this event. But, Rohini is didn't existed during this time, because this event is happened at the start of creation.

2. Rohini is Brahma's granddaughter and who is the real deer beheaded by Shiva

Well, many Puranas mentiones that Rohini is Daksha's daughter. Daksha is Brahma's son, so Rohini is Brahma's granddaughter.

And the deer is actually Yagna Purusha. Here the Sati Khanda from Shiva Purana mentioned it.

36-37. The sacrifice was terrified on hearing the sound, “O I am doomed”, he thought. He was trembling with the legs shaking. His lustre disappeared. He ran in the form of a deer. The heroic Bhadra. with a half curved arrow, decapitated him.

The above reference says that Yajnapurusha was beheaded by Bhadra (Virabhadra), not Lord Shiva himself.

(Note: But, your question is not actually look likes a actual question)

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Ok, that's it.