It isn't very interesting to give flying to your Mesa Pegasus, which already has flying. Anything that would've been able to block the Pegasus when it had flying once will still be able to block it when it has flying twice. On the other hand, it is interesting to give rampage to your Craw Giant, even though it already has rampage 2, as each rampage ability will apply as appropriate, giving the Giant both bonuses whenever more than one creature is assigned to block it.