I will sort by generation:
RBY
In the Japanese version of Blue, you can trade a Kadabra for a Graveler (that evolves into Golem) on Cinnabar Island.
In the Japanese version of Blue, you can trade a Machoke for a Haunter (that evolves into Gengar) in Cerulean City.
In Yellow, you can trade a Marowak for a Machoke (which evolves into Machamp) in the Underground Path (Routes 5 and 6)
DPPt/HGSS
In D/P, you can catch wild Steelix in Victory Road, Iron Island, and Snow point Temple.
In Pt, you can catch wild Steelix in Victory Road and Iron Island.
In D/P/Pt, you can catch Gengar in the Old Chateau if you have any gen 3 game in the gba slot (Dual slot mode).
In HG/SS, you can catch wild Steelix in Cliff Cave and Mount Silver.
In HG/SS, you can trade any Pokemon with Jasmine at the Olivine Gym to receive a Steelix.
BW/B2W2
In B/B2, you can catch Huntail on Route 4 while fishing in rippling water.
In W/W2, you can catch Gorebyss on Route 4 while fishing in rippling water.
In B2/W2, you can trade Emolga for Gigalith (Route 7).
In B/W/B2/W2, you can catch Milotic (which evolves via trading while holding a prism scale in these games and onwards) on Route 1 while fishing in rippling water.
In B2/W2, you can catch wild Steelix in Clay Tunnel, Twist Mountain, and Underground ruins (Dust Cloud).
In B/W, you can catch Politoed in Route 6 while fishing in rippling water.
In B/W/B2/W2, you can catch Politoed in Routes 6, 19, and 20, Floccesy Ranch, Lostlorn Forest, and Victory Road (Cave Entrance) while fishing in rippling water.
In B/W/B2/W2, you can catch Kindra on routs 17 and 18, and P2 Laboratory while fishing in rippling water.
In B/W, you can catch Slowking in Abundant Shrine while surfing in rippling water.
In B2/W2, you can trade Hippowdawn for Alakazam in Accumula Town.
XY
In X, you can catch Huntail in Route 12 while fishing with a Super Rod.
In Y, you can catch Gorebyss in Route 12 while fishing with a Super Rod.
In X/Y, you can trade a Luvdisc in Cyllage City for a Steelix.
In X/Y, you can catch Politoed on Route 19 with a Super Rod.
In X/Y, you can catch Trevenant on Route 20.
SM/USM
In US/UM, you can catch Huntail and Gorebyss in Melemele sea and Route 15 while fishing and by sosing (Huntail at night, Gorebyss at day)
In S/M/US/UM, you can catch Politoed via sosing in Malie Garden at night while it is raining.
In US/UM, you can catch Slowking via sosing in Kala'e Bay.
In S/M, you can trade a Haunter for a Graveler (that evolves into Golem) in Tapu Village.
In S/M, You can encounter Conkeldurr via Island Scan on Poni Island during Mondays.
In S/M/US/UM, you can catch Gengar via sosing in the Abandoned Thrifty Megamart.
In S/M, you can get Trevenant in the poke pelago (after you reach Ula 'Ula island).
In US/UM, you can trade Phantump for a Phantump (which evolves into Trevenant) in Tapu Village.
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Hope I Helped!
