This porphyry tub can be seen at the beginning of the left nave of the Duomo of Milano (sorry for the quality of the pics: I took them with my cell phone). It is now part of a baptismal complex built by Pellegrino Pellegrini in the sixteenth century along with other features inside the Duomo and part of the façade.

I haven't found any documentation on it, except what is written on the baptistry's info plaque. The baptismal basin is actually an ancient tub located previously in the Monastery of San Dionigi, and possibly coming from the Thermae of Emperor Maximilianus Herculeus (II c.), of which four columns are still visible in Milan.

So it does not say it is a sarcophagus, but the dimensions and the similarity to Theodoric's are interesting. There is no lion's head on the side, rather a metal contraption, possibly added later.

For comparison: Theodoric's sacophagus in Ravenna