In Wikipedia, I learned about a deity named Naigamesha, the goat-headed form of Kartikeya.
But, who is he? Is he mentioned in any scriptures?
The Naigamesha form of Kartikeya is a personified graha (disease), but he can be appeased with proper prayer.
Now we shall discourse on the chapter which deals with the medical treatment of (an attack by) Naigamesha...
Offerings of huskless sesamum, garlands of flowers and various dishes should be made to the deity Naigamesha (the preserver of the child) at the foot of a Vata tree on the sixth day of the fortnight and the child should be bathed there at the foot of the tree.
The Mantra runs as follows:—
“May the far-famed god, Naigamesha, the preserver of children, who has a goat’s face with moving brow and rolling eyes and who can assume different forms at will, preserve the child.”
Next chapter (Origins of the grahas) of the same text says:
The Naigamesha Graha who is possessed of a goat's face was created by the goddess Parvati as the friend and protector of the young god Guha and who was as dear to him as his own self.
A similar reference can be found here, in the Nilamata Purana.
Another text that mentions him as a form of Kartikeya (Skanda) is the Linga Purana:
Lord Skanda will be born who will sportingly kill the demon Tāraka. He will be known as Śaktidhara (holder of Śakti or spear), Ṣaḍāsya (six-faced), Dvādaśabhuja (twelve-armed), Senānī (commander-in-chief), Pāvaki (son of ñre), Svāheya (son of Svāhā), Kārttikeya (son of Kṛttikās), Gāṅgeya (son of Gaṅgā), Śaradhāmaja (born in the abode of the forest Śara, lord Śākha, Viśākha, Naigameśa, Senāpati (commander-in-chief), Kumāra, Mahāsena (having a great army). Though he will be only a small boy, he will kill Tāraka and protect Devas.”