Sure! Here are the rewritten sentences with colons and semicolons inserted at suitable places:
1. Some people like to use a fountain pen; others like to use a pen or pencil.
2. The horse is black; it is also a thoroughbred.
3. I think pigs are cute; however, some of them are dirty too.
4. I find cows smelly; I don't go near them.
5. We could sit and work at the library today: it is not crowded.
6. There are two types of leaves: simple and compound.
7. If attacked, you have two options: stay and fight; or turn and run.
8. Platypus is found on the eastern coast of Australia; it is the only egg-laying mammal.
