Various Queries

Could you help me to check if the below sentence is correct?

Today I went to the park and saw some new species of animals. Once I saw that creature, I knew that they have made it.

And, if someone have done something wrong and I would like to tease him about it.
For example, he made an earth which was in the square shape.

Then which one should I say?:
Oh I thought the earth is round!
Oh I thought the earth was round!

If I am talking about something that is still the fact in this moment, can I use past tense and present tense alternatively?

Another example, my friend finished the project on yesterday - creating a high-tech gear. May I use anyone of the below sentences freely?

He created a suit that was able to make a man invisible.
He created a suit that is able to make a man invisible.

Because the time when he successfully created the suit is in the pass, I should be able to use "was". However, because this suit is still here, not destroyed yet, I should be able to use "is" too as it is still present. Am I correct?

Sorry, my statements are really ambiguous.

Thank you everyone!