“Mendicants, dragons reproduce in these four ways. What four? Dragons are born from eggs, from a womb, from moisture, or spontaneously. These are the four ways that dragons reproduce.”
The “dragon” (nāga) is a powerful (Bu Pc 51:1.4) serpentine (SN 1.37:7.4, Cp 20:1.2) entity with fiery breath (SN 3.1:8.2), bearing magical gems (Bu Ss 6:1.3.2, Ja 253). They dwell underground (MN 23:2.39), or in the ocean (Ud 5.5:17.1), or in the mountains (SN 46.1:1.2), guarding treasure (Ja 506) that they may have stolen (Kp 8:4.3). They may be man-eaters (SN 6.4:9.2) or protectors (Ud 2.1:2.2), and occasionally had to be engaged in magical combat (Kd 1:15.2.3), Bu Pc 51:1.4). They are color-changers (SN 3.1:8.1, Cp 13:3.1) and shape-shifters who, though they revert when they sleep, can temporarily take human form (Kd 1:63.1.1), even for the purpose of intimate relations (Bu Pj 1:10.14.1). The line between the supernatural entity and organic snakes is as fluid as a serpent’s form; dragons act as the leaders and protectors of all snakes (Kd 6:23.13.5), while poorly behaved dragons may be banished to a snake’s existence (Ja 304). Powerful and feared though they are, they are still no match for the cruelest of predators, man (Ja 524). For a history of the word, see SN 46.1:1.2.
These four are defined at MN 12:32.1; see note there for Vedic and Jain sources. The Dhammic import of this idea is that even such powerful entities are born, and hence are subject to old age and death.