What does giant horse mean?

Because all the giant horses are dead, exactly one third.

It’s actually a homophone: the giant’s horses are all dead—the spoiler’s (people’s) mothers are all dead.

Joke background: On April 24, 2019, "Avengers 4" was released in mainland China (the earliest in the world), with a box office of 188 million yuan. After watching it, many people posted on WeChat Moments and Weibo to talk about which hero. Death and the like, basically the spoilers are over. Many people wake up early in the morning and check their WeChat Moments and find all kinds of spoilers, so they start to ridicule the spoilers.