Lying is very common in young children, for a variety of reasons. In fact, I see it as somewhat of a milestone. Its not right, but it shows they are problem solving, showing imagination, recognizing social norms, etc. I know as adults we see lying as a huge issue, but really for kids, it doesn't pack the same punch. I don't think they have a firm understanding of how it affects other people yet. They are developmentally still very self-oriented. So first, I would advise trying to separate your emotions from it because coming down really hard on it can give the lie power (which you don't want) and/or give motivation for the child to learn to be a better liar so they won't get caught next time.
So in the case of the stealing of frogs, he obviously recognized that what he did was wrong, and lied to try to get out of trouble. Not a good thing, but also a very normal reaction from a child who is still figuring the world out. I think having him take the toy back and apologize is a good thing, but also make sure you are there as a SUPPORT for this, not as an added pressure. When I have my 5 year old apologize to an adult, she gets really scared and is embarrassed. I hold her hand, encourage her to do it, and tell her how proud I am of her afterward.
I personally believe encouraging the truth is better than over-punishing a lie. You want them to feel comfortable telling you the truth in the future. Good luck! this parenting stuff is hard!