Enkidu manifested as a Lancer-class Servant because of his Noble Phantasm.
Though calling it a Noble Phantasm wasn't entirely accurate.
Enkidu's very essence was to preserve the bond between gods and humanity.
According to one account, Gilgamesh was born into the world as a bond between gods and humans, so that the gods would not lose their power and humanity would continue to worship them as gods.
But whether he had forgotten his mission or deliberately ignored it, the Hero King failed to fulfill his duty. Instead, he began to rule in a way that drove gods and humans apart.
As a spear thrust into the Hero King to correct him, question him, and hold him accountable for failing his mission—in other words, as a divine weapon created to pierce one who had turned against the gods and return the bond to their hands—Enkidu was highly likely to be chosen by the Holy Grail as a Lancer.
From a certain perspective, Lancers had never truly been beings who wielded long spears.
For these Heroic Spirits, even the mere concept of a spear could be enough.
Much like the origin of Lin Yun's Berserker, all it took was for someone to look like a barbarian or a berserker, or even just to seem a little wild in battle.
Add in the fact that the greenskins under his command were incredibly savage, and he was granted the ability of Mad Enhancement. From one perspective, that was rather arbitrary.
In a sense, Enkidu's Noble Phantasm was something like a spear. In other words, the blow he delivered by using his own body as a weapon was the truest expression of a Lancer.
He did nothing more than pierce and stitch the world back together.
If a barrier existed in the rift between heaven