Wither Eggs adds eggs for the Wither, Ender_Dragon, Iron_Golem, Snow_Golem, Illusioner and Giant, currently these seem to not be dropped by the Capturing enchant.
This might have something to do with that mod using ForgeSpawnEggItem rather than SpawnEggItem which Apotheosis uses, as seen here: WitherEggs.java

yeah looks like there's finally a util method to get the egg by the id, i'll move it over.