When I throw a fishing rod bobber near a cow, It will homming to cow and stick to it, but only visually. By checking entitydata of bobber or by placing wooden pressure plates near the cow, you can see that bobber is invisible next to entity. When it happens, You can't pull "catched" entity.
In attachment you can see one triggered pressure plate (there bobber landed), when cow went further with visually sticked fishing line.