I guess maybe Delibird uses rapid spin by doing an ice skating spin maybe? Like maybe it can freeze the ground under it's feet and use it to figure skate!
Wow I'm really stretching there.
But as for Cryogonal and Gigalith learning it, I can rationalize that as them storing the solar energy in their crystalline structures. Fire types getting it is much like water types getting ice beam, when you really think about it, it doesn't make sense but it's basically standard. As for Mawile, Aggron, Nincada, Gulpin, and Altaria... I'm at a loss. I guess I can rationalize Aggron and Altaria as being dragon-like things so shit, why not give them super powered sun lasers for rule of cool reasons, but the others? I just... no.
Looked up some other Solar Beam users too... Venonat and Venomoth, A crapload of normal types (those can be explained weakly using my "normal is the base type so they can learn stuff from all types" explanation), Ledyba and Ledian, Spinarak and Ariados, Natu and Xatu, Yanma and Yanmega, Beautifly and Dustox, Surskit and Masquerian, Trapinch familty, Lati@s, Rayquaza, Deoxys, Wormadam and Mothim,Togekiss, Uxie (but not Mesprit and Azelf, oh no that would be silly), Cresselia, Venipede family, Dewbble family, Sigilyph, GARBADOR and GOLURK!!?!??!?.
Ok, it seriously looks like Gamefreak just hands out Solarbeam to whatever the fuck they feel like giving it to.