I'm interested in this because my 7 year old has been telling more lies lately and I feel torn as to how to view it and how to handle it.
My son has such a vivid imagination. He will often begin by telling me something I know is truthful but it quickly escalates into an exaggerated and obviously fantasy based story. The thing is I'm not even sure my son knows where or when it crosses from reality into fantasy. I can tell he's seeing it all in his head as if it's real. If I ask him if it's real he will say it is. Is this lying? I've started to talk to him about his 'imagination world' and rather than accuse him of lying I'll say, 'is this from your imagination world?'
However, he also does the more obvious kind of lying that you're talking about where I'll directly ask him if he's done something and he will lie. He'll even continue to lie if I ask him several more times. I did take away his pocket money recently because I was so exasperated by it!
The reason I mentioned the imagination stuff is because it links to something I remember reading once about children and lies. It said that a child's truth is often what they wholeheartedly want the truth to be rather than our very fixed notion of the truth. So when we ask a child if they have done something which they know they should have done or that we would want them to have done, their 'truth' in that moment is that they wish they had done it. It said that the line between fantasy and reality is so much thinner for children that things don't always look the same way to them. I suppose that looked at in the most positive light, they wish so much it was different that they make it so in their mind. I think it also said that children will always strive for things to feel good and right and joyful, and I guess admitting that you've messed up and maybe getting in trouble doesn't lead to any of those feelings!
Personally I think that losing a pet over a lie would be too harsh. If I feel I've made a wrong decision in the heat of the moment I don't mind admitting that and explaining that I wasn't thinking clearly because I was upset and angry. (In the toddler years I have retrieved and washed things from the bin that I threw away in a less than perfect parenting moment and returned them with an apology!) Personally I think this is a more useful message than blanket consistency. I agree with meemee that a talk about the lizard is probably a good idea. Does she still want the lizard? Is the responsibility feeling too much? My 7 year old feeds the cats most days but I don't think I'd want him to feel a life or death responsibility for them.
I don't know if this would work for your daughter but when I feel there is something important which needs to change I will usually ask my son what he thinks a good solution would be. Perhaps when you're feeling calm you could talk again about the impact of lying about feeding a pet and how it makes you feel very worried about the lizard. You could say that you don't know what to do now and that is why you said you wanted the lizard to go to a new home. You could then ask your daughter whether she has any ideas about what to do in this situation. My son often comes up with solutions I wouldn't have thought of. He seems to take things more seriously when I include him in the problem solving too. Most of our household rules have been formed this way.
In a more general sense I think that experimenting with lying is probably pretty natural. Lies are one of those things that are not as black and white as they may first appear. For example we teach that 'lying is wrong' but if a child was asked 'do you think I look nice in my new dress?' and they replied honestly, 'no, I think it's ugly and doesn't suit you' we would probably tell them off for rudeness, despite the fact they would have had to lie to be polite. Or if they didn't like a present they were given and said 'I don't like it' instead of 'thank you', again the honesty would not be rewarded where as a lie would have been. Lies are a complex social issue!
Good luck with this and if you have any brainwaves please let me know!!