Pegasus does not go into the backow for one reason: Negation of summon.
When a monster summon is negated it was never considered to be on the field, so when the card says "If this face-up card is destroyed while it is in a Monster Card Zone,..." It was never there so it cannot get this effect. And you do not get the first effect of putting a crystal beast in your s/t zone.
______________________
