Support for input using SI prefixes was [added to the Monero codebase](https://github.com/monero-project/monero/pull/1826) on March 3, 2017 by [Moneromooo](https://github.com/moneromooo-monero). The smallest unit of Monero (10^-12 XMR) was originally called a tacoshi in honor of user [Tacotime](https://bitcointalk.org/index.php?action=profile;u=19270), an early Monero contributor and was later renamed for ease of use and consistancy.
Monero denomination names add SI prefixes after dropping the initial "mo" for ease of use. Actually, the smallest unit of Monero is 1 piconero (0.000000000001 XMR).