Atreyu and Artax
The NeverEnding Story (1984)

Tasked with nothing less than curing the dying Childlike Empress, Atreyu sets out to defeat The Nothing and save her. His only companion is his trusty steed, Artax. Their journey is fraught with peril, and the movie's most heartbreaking moment comes in the Swamps of Sadness as Artax sinks slowly, excruciatingly into the mud and dies.