"Sharp Tooth", the main antagonist in The Land Before Time series, isn't a T-Rex; rather, it's a Carcharodontosauridae Species broken off as an early iteration from Allosaurus/Allosauroidea.
1. Carcharodontosauridae translates to "Shark-Toothed Lizard", as they were named due to the similarity of their teeth to the Genus that The Great White Shark belongs to, Carcharodon: Greek for "Sharp Tooth"
2. It is hotly debated that T-Rex may have been mainly a scavenger. Regardless of that, Carcharodontids are universally recognized as hunters, primarily hunting Sauropods.
3. In multiple instances, Sharp Tooth is seen hunting in packs - of which it is suggested that Carcharodontosauridae, such as the later, Giganotosaurus, did.
4. Little Foot is an Apatosaurus. Apatosaurus didn't live during T.Rex's time - they died out roughly 145million years ago, whereas T.Rex didn't live until roughly 80million years ago. Allosaurus, however, lived roughly between 163-96million years ago.
5. Allosaurus is seen in The Land Before Time 4 as the Brown-Ridge Sharp Tooth.
6. Giganotosaurus is seen in The Land Before Time 5 and is referred to as "The Plated Sharp Tooth".

My theory is that "Sharp Tooth" in general are various species of early Carcharodontosauridae as they were differentiating within the Allosauroidea Superfamily. None of them were T.Rex at all, as it's an entirely different family.
I'm convinced - there's no way a cartoon could be so scientifically inaccurate as to botch this one up; we were all just idiots as children.