small pig, sucking pig, to shuffle along without lifting the feet
㹠 centers on a small or young pig, especially one still suckling. The meaning of shuffling along without lifting the feet is likely an extended sense based on the pig's movement.
㹠 combines the animal radical 犭 with 屯, which may contribute a phonetic element. The exact historical development is uncertain, and the character is rare in modern Japanese.
The left side 犭 suggests an animal, and the right side 屯 looks like a curled-up creature. Picture a tiny piglet curled up, shuffling without lifting its feet.
For トン, imagine a small pig weighing one ton: ton -> トン, and the piglet shuffles heavily despite its size.