Fire Lad often breathes fire by accident, since there's a chance of it happening any time he exhales. If he sneezes?
Run.
The "giant turtle" there references Jimmy Olsen's Silver Age monstrous alter ego, the Giant Turtle Man. While never a formal member of the Legion of Substitute Heroes, there is an infamous Silver Age story where LoSH member Colossal Boy dresses up as the Giant Turtle Man so he can, more or less, just screw around with Jimmy's head.
In the post-Crisis era, I believe the Legion picked up a Reservist called Turtle who has super-invulnerable that homaged the old Giant Turtle Man. Another reservist from that run was perennial Legion Substitute Night Girl, who has super-strength and invulnerability... but only when it's dark.