Mikey the Robot "sees the frog" using the Pixy camera vision system. Pixy costs about $70. and easily interfaces with an included cable to Arduino. Pixy can be "trained" to recognize up to seven different colors. Pixy returns the x,y coordinates of the center of a trained object, along with the width and height of the object. Pixy is open source and all the hardware and software are available through cmucam.org.
In my setup, Pixy communicates with an Arduino which I call a "Sensor Interface." When the center of the target (frog) is toward the right (>170 out of 319), the red LED illuminates and the robot turns toward the right. When the frog is toward the left (<150 out of 319), the white LED illuminates and the robot turns to the left. When the frog is pretty much in the center (>150 and <170), both LEDs illuminate and the robot moves forward.
4WD Arduino Compatible Mobile Platform--Makershed.com
Arduino Motor Shield--Makershed.com
(2) Arduino Uno--Amazon.com
(7) NiMh AA batteries--Amazon.com
Misc: Red led, white led, (2) 220 ohm resistors, SPST toggle switch