Flame emission (or flamethrower) is a hazard introduced in HTFA 5. It first appears to be a small flame, but wait long enough and the flame briefly bursts into a long line of blaze which can hurt you if you don't avoid it on time.
This kind of hazard can either appear from lava pools, gray Bowser statues, or any other special means of spewing flame.
A stronger version of this is introduced in HTFA 6. Instead of unleashing a thin line of fire and then "retracting" itself, this variant bursts upwards and can reach higher than the usual flame emission. Also, it deals two points of damage instead of one.