Yeah, there are six different Experience Point formulas that are used...
Erratic: 125,000 exp. points to level 50; 600,000 exp. points to level 100.
Fast: 100,000 exp. points to level 50; 800,000 exp. points to level 100.
Medium-Fast: 125,000 exp. points to level 50; 1,000,000 exp. points to level 100.
Medium-Slow: 117,360 exp. points to level 50; 1,059,860 exp. points to level 100.
Slow: 156,250 exp. points to level 50; 1,250,000 exp. points to level 100.
Fluctuating: 142,500 exp. points to level 50; 1,640,000 exp. points to level 100.
Erratic and Fluctuating were introduced in the 3rd generation, but there aren't any 5th generation Pokemon that use those formulas.
Erratic starts out leveling up slowly, but once the Pokemon reaches level 50, it'll require less experience to level up than the Pokemon that use the Medium-Fast formula. As for Fluctuating, the Pokemon starts out leveling up very quickly, but as the Pokemon reaches level 50 and beyond, it gets much harder to level it up.
You can easily train a Zangoose to level 100 in just one day, as it only requires 600,000 exp. points to reach that level. Seviper, on the other hand, can't reach level 100 that quickly, requiring 1,640,000 exp. points to reach that point.