A single Pokemon with varying stats - the happier it is when it evolves, the more skewed its stats are towards Speed and less towards bulk, and vice versa.
A Pokemon that only evolves when it's on the same team as its own evolved form. A hundred or so of the evolved form are given out at launch.
An egg-themed Pokemon (besides Togepi, Eggsecute, etc.) that evolves when you walk some exorbitant number of additional steps.
A Pokemon that only evolves if it's the only Pokemon in your party.
A Pokemon that only evolves if you white out - if it got enough XP to gain a level, then fainted along with your other Pokemon.
Two-to-six shitty Pokemon that join together Voltron style if they all level up at once. IVs, natures, etc. could be taken highest-of-all, averaged out, or taken by a predetermined formula (big dumb one gives HP and physical stats, tiny smart one gives Speed and special stats, leader gives nature and nickname, etc.)
Pokemon that evolves differently if you cancel its evolution enough times - for example, evolving into Evolution A from levels 20-39, and Evolution B from levels 40 and on, or odd or even levels, or if you waste 10 Thunder Stones before letting it evolve.
A Pokemon that evolves if your character is wealthy enough.
A Pokemon that evolves if it's fathered/mothered enough eggs.
A Pokemon that only evolves in sets - if you have two-to-six of the same Pokemon in your party, one evolves into each possible form.
A Pokemon (with Color Change, Multitype, Protean, Conversion(2), Reflect Type, etc.) that evolves only if it has been every type at least once.
A Pokemon that has different forms if evolved in completely absurd times - so if you set your system clock to Y2K, it gains the Electric type, or during the 1600s makes it a Steel type with a knight's helmet.
