From the Super Mario Wiki
Terrapins are armored Koopas with green shells that appear in Super Mario RPG: Legend of the Seven Stars. In the game, they are the weakest soldiers in the Koopa Troop, even weaker than a Goomba. A similar (and much stronger) enemy is the Terra Cotta. The word "terrapin" is an ambiguous term referring to turtles, which Koopas are quite similar to.
Terrapins fight solo or in groups of two to four and attack their opponents by punching, which does not do much damage. They patrolled the halls of Bowser's Keep in search of intruders, and appear mostly in the very beginning of the game, before Exor crashes into Bowser's Keep. They only had 10 HP and 1 attack power, and 8 defense; this made battling them good training for Mario at that point in the game. Mario also gains no EXP for defeating them.
Terrapins are the precursors to the Koopatrols of the Paper Mario series. However, Terrapins and Koopatrols themselves are very different from each other in stats, although their main purposes are identical in that they are both armor-clad Koopas who guard Bowser's Castle.
Names in other languages
|Koopa Soldier; from the Japanese name for Koopa Troopa (Nokonoko), and 「兵」 hei meaning "soldier".