The Turtle is a yellow turtle-shaped icon that appears on the Prius' dashboard when normal battery power is not available for some reason. The typical reason for its appearance is when excessive acceleration has depleted the battery. However, it can also appear when extremely cold temperatures temporarily reduce the battery's performance. In any case, acceleration is supposedly limited to whatever the gas engine can produce, until the battery is sufficiently charged and/or warmed to provide normal power.

My turtle appeared because of cold weather. When I started the car in the morning,
the Outside Temp read 1°F, and the battery icon showed a normal 3/4 charge.
The turtle remained displayed for 6 mi of mostly highway driving (55 mph).
However, I observed no noticeable reduction in acceleration or power;
although I didn't tax the car excessively.