Shelgon and Salamence are both Dragon-type Pokemon. They are both usable, although Shelgon is never seen. That is because people think that it is not strong enough. Well, look at this:
Shelgon
Rock Head Recoil moves have no recoil. Type Tier Dragon NFE Statistics
Min- Min Max Max+ HP 65
- 271 334 - Atk 95
203 226 289 317 Def 100
212 236 299 328 SpA 60
140 156 219 240 SpD 50
122 136 199 218 Spe 50
122 136 199 218BST: 430
Salamence
Intimidate Lowers the foe's Attack 1 stage. Decreases wild encounter rate. Type Tier Dragon / Flying OU Statistics
Min- Min Max Max+ HP 95
- 331 394 - Atk 135
275 306 369 405 Def 80
176 196 259 284 SpA 110
230 256 319 350 SpD 80
176 196 259 284 Spe 100
212 236 299 328BST: 600
620-430=190
So basically, Shelgon is not shabby.
Weaknesses:
Shelgon: Ice, Dragon
Salamence: Ice, Rock, Dragon
Resistances:
Shelgon: Fire, Water, Electric, Grass
Salamence: Fire, Water, Grass, Fighting, Ground, Bug
Salamence has one more weakness but one more resistance, making it pretty much a tie in typing.
Final Tiers:
Shelgon: UU
Salamence: OU

Rock Head Recoil moves have no recoil. Type Tier Dragon NFE Statistics
Min- Min Max Max+ HP 65
- 271 334 - Atk 95
203 226 289 317 Def 100
212 236 299 328 SpA 60
140 156 219 240 SpD 50
122 136 199 218 Spe 50
122 136 199 218BST: 430

Intimidate Lowers the foe's Attack 1 stage. Decreases wild encounter rate. Type Tier Dragon / Flying OU Statistics
Min- Min Max Max+ HP 95
- 331 394 - Atk 135
275 306 369 405 Def 80
176 196 259 284 SpA 110
230 256 319 350 SpD 80
176 196 259 284 Spe 100
212 236 299 328BST: 600
620-430=190
So basically, Shelgon is not shabby.
Weaknesses:
Shelgon: Ice, Dragon
Salamence: Ice, Rock, Dragon
Resistances:
Shelgon: Fire, Water, Electric, Grass
Salamence: Fire, Water, Grass, Fighting, Ground, Bug
Salamence has one more weakness but one more resistance, making it pretty much a tie in typing.
Final Tiers:
Shelgon: UU
Salamence: OU