So... I'm DMing a group through Tomb of Annihilation, and they early on took on the services of the Guide Eku. As some of you might know, Eku is in fact, not a human, but a Coatl who has willingly polymorphed into human form.
Eku took some damage in a fight, and had her true form revealed since she was knocked out of her human form. The group accepted her, as Coatls are known to be Celestial servants, and now she has become sort of an emotional centre of the group, and the players constantly question her origin.
So I searched for a plausible origin for her, and as I scanned through a Wiki-page I found the name of another Coatl, Ecatzin. So... a Coatl, linked to Chult, with a similar name/nickname? Seems to me like there is more to this than a coincidence.

Almost nothing is said about Eku's background, but I was wondering if anyone else has come across this thread of thought? Could she and Ecatzin, who led the Tabaxi from Katashaka to Chult, be the same Coatl but more than 4000 years later? They are supposed to be unaffected by the passage of time, so...?

I am in a very similar situation and would love to find some info for her background.

So I don't know anything about Ecatzin other than that it served Ubtao, so let's work with that. I have a basic story, and you can fill it out as you want.

We know the Couatl were once the Sarrukh, one of Toril's Creator Races, who left Faerûn behind, and was transformed into the Couatl by Jazirian. Eku could have been a Sarrukh that was left behind on Abeir-Toril. Left to wander the land (Sarrukh livespans are around 1000 years), she eventually came to Chult, ruled by the primordial-turned-god Ubtao. Eku was tired at this point, and half mad from loneliness, yet she could not die. Ubtao took pity on her, and, being a god, he knew of what had happened to the Sarrukh that left her behind, and granted her the same power. 

When Ubtao turned his back on Chult, she was again left lonely, and devolved completely into madness. In her madness, she started seeking out the wicked to punish them, as punishing a wicked priest of Bane who had come to Chult had been the last task Ubtao gave her. Her search for wickedness brought her to Omu. She could not defeat the trickster spirits' combined power on her own (Couatls are actually pretty weak), so she traveled to the astral plane to find someone who could. There, she met Acererak, and asked him, and you know the rest. When she saw the horrors of what Acererak did, it snapped her out of her madness, but it was too late. Now, she seeks to atone by working against Acererak's vile deeds, no matter the cost.