ditto
Snivey evolves into Servine at Lv. 17.
Level 17.
No, it is only in Pokemon Black/White/Black 2/White 2.
NO
The only way to find a Snivy is to get it from the starter Pokemon. If not, then you'll have to get it over the Global Trade.
First you need to be able to access the Daycare Centre which is on the route between Wellspring Cave and Straiton. Snivy is in the Field/Grass groups so there are large variety Pokemon it can breed with, especially with Ditto (which 99% of Pokemon can breed with the exclusions being Legendaries and Nidoqueen)
Get someone to breed one on their version, and trade it to you.
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Snivey
You can't catch a Snivy in the wild, but you can transfer it from Black and White or take a chance and try to get one on the GTS
Tepig evolves into Pignite Into Emboar and snivey evolves into servine into serperior and oshawott evolves into dewott into daikenki
Any Pokemon that can breed can breed with a ditto.