Robot Frong (name given in Tiger Electronics' Strider manual) is a minor stage enemy exclusive of the NES version Strider. "Frong" is likely a typo on the word "Frog", probably in reference to the enemy's constant jumping.
The Robot Frong is a four-legged machine guard used as a defense robot in several areas. It has a small shell-like body with what appears to be an eye in its front and four spider legs for motion. It also appears to possess a brain as its central core, which comes out after the body's destruction and moves by itself with a similar set of spider legs.
The first form walks along and, once it closes on Hiryu, stays in place and starts to continously throw bombs in an overhead arc at his position. The bombs can damage either by contact or through their explosion once they hit the ground. The second form has a simpler attack, only jumping around and trying to damage Hiryu by direct contact.
Both are difficult to approach and destroy, specially the core as its both faster and a smaller target.