The Sentinel XS is a recurring car throughout GTA:VC (VCS) and GTA 3 (LCS), however it is not present in San Andreas with the introduction of tuning shops. The default Sentinel however offers a very limited amount of customizations.
This is not a conversion from another game. This is totally made from scratch. It's the default Sentinel with parts that match the old renditions of the XS, a two body color map, new wheels, and a few cosmetic features.
For more info and comparisons, see screenshots.