Tar is a sticky molten form of asphalt. It is famous for trapping animals and appears in the first and the third The Land Before Time films.
In the Movies
The tar has appeared in two Land Before Time movies.
In The Land Before Time, it appears in the Mountains that Burn. It's presumably forming because the rivers of lava are melting natural asphalt in the ground. This tar was likely included because at that time, tar was commonly associated with the era that dinosaurs lived in (similar to volcanoes). The association between prehistoric times and tar is likely due to the La Brea tar pits in California (despite the fact that the La Brea tar pits were a lot later than dinosaurs, so mainly caught creatures like mammoths). Therefore, the ones in this movie could be considered as a reference to the La Brea tar pits. Also the inaccuracy is, tar pits did not form in the Mesozoic era (the age of dinosaurs)-only in the Cenozoic (the age of mammals).
When the gang (minus Littlefoot) crosses through Petrie gets stuck in the tar after falling off Cera's back. Cera doesn't seem to notice but luckily Littlefoot, Ducky and Spike arrive and try to help Petrie escape (after Littlefoot saves Ducky and Spike from being trapped on a lava river). Unfortunately, they end up in the tar pit as well. They somehow escape offscreen, covered in tar.
Meanwhile, Cera is being menaced by a herd of territorial Domeheads. Then a tar monster that towers over the Domeheads approaches. The Domeheads flee, terrified and the monster lifts Cera, who is frozen with fear, into the air. However, it turns out that the tar monster is only the rest of the gang, aided by some bones and branches.
It is covered by a layer of water. Hyp and his friends find it while searching for water and mistake it water. Hyp jumps in and begins to drink but gets stuck. Luckily, the gang arrive and with help from Nod and Mutt pull him out.