There are lots of guys with underwhelming rookie seasons (Fox, Bam, Jaylen Brown, etc), but the only recent (meaning the last 20 years) guys I've seen that started just HORRENDOUS and turned into something are
1. Andrew Bynum
2. Sabonis
3. Monk
4. Kris Dunn
5. Thabo
There are caveats to each of these guys other than Sabonis and Thabo though
Bynum: Entered the league at 17 years old, youngest player ever
Monk: Might have been battling a drug addiction at this time?
Kris Dunn: Flushed out of the league and needed multiple years to make it back.
Sabonis is the clearest case of a guy just looking like one of the worst players in the NBA as a rookie and then turning it around and becoming a star though.