A coffin in Ultima I
Vasculio's old coffin in Serpent Isle
In the time of Ultima I, coffins were often found inside the dungeons. However, they weren't used for holding the bodies of the dead. Instead they basically acted like treasure chests, containing riches. However, most were trapped and needed to be opened with the fitting magic spell in order to get the contents.
After that time, the forces of evil no longer used coffins to store their riches and the dead of the lands were buried timely, so that coffins were never encountered above ground as well. However, in Ultima VII Part Two, the burial rites of the Ophidians resulted in many coffins being found in the various catacombs. These coffins were little more than wooden boxes with little to no individual design.
Also, big ornamental coffins were found like the one of Beatrix and the two coffins Vasculio was using for his rest in his vampiric state.