Emercoin (EMC) is the mineable coin based on the Emercoin’s own blockchain. You can convert these coins to other cryptos and back on a few exchanges. Emercoin was launched as early as in 2013 as the Peercoin’s fork.
Tether is a cryptocurrency launched in 2014. It is a so-called stablecoin currently leading by market capitalization. Although according to the initial idea, each Tether was backed by $1 (that was going to keep the price of Tether stable), the company not always managed to maintain this price (at some point in October 2018 the price dropped to $0.9 per one Tether). Tether succeeds in terms of market capitalization and its price is almost all the time is really close to $1.