u/shrimp_with_anxiety

Hi, I’m looking for an old 2D animated movie I saw as a kid (~2007 maybe earlier). It was on DVD.
Two main characters, animals/creatures (maybe deer, llama or dinosaur-like), long necks, simple round heads, very flat design, each a single solid color, I think one was orange and another one green or maybe both orange.
Dark forest setting that feels like a closed loop. They go into a cave/underground area. Plants/roots become alive and trap them. One gets dragged underground and disappears. I also remember a disturbing moment like a limb being bitten off. Colors sometimes shift to black/red for fear.
Not sure if it was a full movie or DVD anthology. My memory may mix deer/llama/dinosaur.
If anyone recognizes this, please help.