I should also point out that Bel is in fact a unique Baatezu, not just a big pit fiend, though 3e hasn't gone out of its way to depict him as such, and this probably owes a lot to the way that Guide to Hell handled the situation (I have reason to not like portions of that book). As detailed in PS, Bel may have started out as a lemure and worked his way up to pit fiend status, and been a pit fiend for some time, but his seizure of power from Zariel and approval of that coup by both the Dark 8 and Asmodeus himself made Bel something else entirety. His avatar in 2e as described by Colin McComb was more devilish humanoid than pit fiend, but 3e has tended to almost exclusively model his appearance on that of just a much bigger pit fiend, which is unfortunate.