They are "Promotional or Potion" frogs. you won't find them in the pond, in presents or in the games. you either have to wait for them to appear in the FrogMart (which is repeating the same small number of frogs over and over because of a known bug in the random number generator) or you have to get them as a gift from another user. that's why people are asking for them all the time. on the 'breed' page here on this wiki, the frogs with an asterisk after their names are POP frogs.