Form DifferencesAs like the Mainstream Pokemon Games, various Pokemon in Pokemon GO have alternate forms and appearance differences. Even though this changes how one Pokemon of a given Species looks to the next found in yet that same Species, these differences have no significant value within the game.
One of these differences revolve around the Gender of a Pokemon, being first introduced in the 2nd Generation Update (identified on the Pokemon's Profile). Most Pokemon have two genders, but some Pokemon are either exclusively male, exclusively female, or have no gender altogether. Most Pokemon don't have any appearance differences from male to female, but there are a number of Pokemon that have aesthetic differences from one gender to the next.
The following Pokemon have Gender Differences in Pokemon GO: venusaur, butterfree, rattata, raticate, pikachu, raichu, zubat, golbat, gloom, vileplume, kadabra, alakazam, doduo, dodrio, hypno, rhyhorn, rhydon, goldeen, seaking, scyther, magikarp, gyarados, meganium, ledyba, ledian, xatu, sudowoodo, politoed, aipom, wooper, quagsire, murkrow, wobbuffet, girafarig, gligar, steelix, scizor, heracross, sneasel, ursaring, piloswine, octillery, houndoom, donphan, torchic, combusken, blaziken, beautifly, dustox, ludicolo, nuzleaf, shiftry, meditite, medicham, roselia, gulpin, swalot, numel, camerupt, cacturne, milotic & relicanth
Two Pokemon specifically, unown and castform, have different forms within their given Species. Unown actually has 28 different forms, ones that take on the appearance of each letter apart of the English Alphabet (as well as "!" and "?" punctuations). Each form is determined randomly when Unown appears, but there is nothing special about collecting all of its forms besides a special medal received for such an achievement.
As for castform, it's form changes depending on the weather you find it in within Pokemon GO: in Cloudy, Partly Cloudy or Windy Weather, you will find Castform in its Normal Form; in Clear Weather, you will find it in its Sunny Form; in Rainy Weather, you will find it in its Rainy Form; and in Snowy or Foggy Weather, you will find Castform in its Snowy Form. Unlike the Mainstream Pokemon Games, Castform will not change forms in Pokemon GO after being captured. Much like unown and its given forms, there is no difference with Castform's different forms, except for the fact its Typing is different with each.
The final Pokemon that can have appearance differences are pichu, pikachu and raichu. Outside of their normal forms, these three Pokemon can be found and obtained wearing a special hat for a given promotion that was previously run. At the present time, there are only four hats for these Special Event Pichu, Pikachu and Raichu: Santa Hat, Party Hat (Pikachu & Raichu Only), Ash Ketchum Hat, and Witch Hat.