I could eat a horse: This is an idiomatic expression. You must use ‘COULD’ (NOT can) when you don’t really mean what you say. We don’t mean that one will eat a horse, it is an exaggerated statement. The following situations relate to ‘NOW’ (i.e. present situations)

• I could eat a horse! (i.e. I am so hungry and I feel like eating a very large amount of food.)  -(present)
• He is so angry with her. He could kill her! ( i.e. present)
• He could beat me! (i.e. He has lost his temper so anything may happen unpredictably)
• The MD is in a very angry mood. You had better not put up the request now. He could shout at you!
• He is so tired. He could sleep for a week!

Past situations

When we want to convert the above examples into the past the following structural changes will take place:

(Could have + past participle of the verb)

• I could have eaten a horse!  (i.e. I was terribly hungry, maybe after a tiresome trekking)
• He was so angry with her. He could have killed her! (past – actually he felt like doing anything like killing her, but he didn’t do so)
• He could have beaten me! (i.e. he was in such an angry mood)
• The MD could have shouted at me! He was in a very bad temper.
• He was so tired. He could have slept for a week! (Maybe after a long trip –  a past situation)