Also spelt 'Tecuciztecal' or 'Tecuciztecatl' (that being Google's suggestion), Tecciztecatl was an Aztec god; a lunar diety whose name means 'old moon god'. He represented the Man in the moon.
Tecciztecatl was the son of Tlaloc and Chalchiuhtlicue, and was originally the god of snails and worms, a fairly undesirable position. In order to improve his station in life, he volunteered to leap into the sacrificial flames to become the sun, but then he chickened out and Nanahuatzin jumped in ahead of him.
Tecciztecatl quickly followed Nanauatzin in, but it was too late. An eagle swooped down and took them both up into the sky, where Nanahuatzin became the sun and the unfortunate Tecciztecatl was thrown to the moon1.
Later, Tecciztecatl changed his name to Metzli.
He is sometimes depicted with butterfly wings, and sometimes as an old man carrying a large seashell (the moon) on his back.
1 Note that some versions of the story say he became the moon. Additionally, he is sometimes said to have been thrown by Nanahuatzin himself. And to top it all off, one version I've seen says he was in the form of a rabbit, while another says he had rabbits thrown at him by the other gods as they taunted him.