WHIRLWIND EXPLANATION: Pokémon have a fixed, permanent score from 0 to 31 for each stat that affects how good that stat can ever get.

This calculator will figure out that score (called a "gene" or, more obtusely, an "IV") for you, so you can discard the unworthy. It's more accurate for higher-level Pokémon, so it's helpful to go into a level 100 wifi battle with someone and check your Pokémon's stats from there.

Stat calculator

Rhydon
Rhydon 		Base
stats		 Stats Effort Stats Effort Possible genes (IVs)
Level Level
HP 105
146–160
2–3
Attack 130
119–133
31
Defense 120
110–124
20–21
Special Attack 45
44–58
31
Special Defense 45
44–58
29–30
Speed 40
40–53
2–4

Hmm, I need more information to figure out your Pokémon's genes exactly. Try raising it to level 45 and entering its new stats.
Remember: if your Pokémon battles at all, you'll need to track the effort it gains. Consider saving your game and using Rare Candy instead.

And for your copy/pasting pleasure:

2–3 HP; 31/20–21 Physical; 31/29–30 Special; 2–4 Speed

or start over