I expected this. Reasoning (using only what we knew).
1. He gives you a shiny Gible in at least one version. 2. He has Garchomp on his team in Black 2 only. 3. He has a Dragonite in it's place in White 2. 4. Therefore, he would probably give you a Shiny Dratini instead in White 2.
Yog-Sothoth knows the gate. Yog-Sothoth is the gate. Yog-Sothoth is the key and guardian of the gate. Past, present, future, all are one in Yog-Sothoth.