If you kill level 100 goblin, I say you deserve a dragon. Heck, if you can even find level 100 goblin, you might get one just for the discovery...
Might want to check out pet guides. Here is my favorite for some reason. :p
http://forum.us.runesofmagic.com/showthread.php?t=53601
As for the question, what makes egg good is rarity (Holy is what you should be aiming at), element (depends on your class), aptitude (the closer to 100 the better) and level (most people should start at low level and grow the pets). In other words, you should not aim for high level pet to start with. Look for the low and grow him. You will end up with better pet.
The relationship between level of the mob and level of the pet is directly proportional up to a point. The higher level the mob, the higher level the potential max level of the egg is. You don't automatically get high level egg from high level mob, you just have a chance to do so.
And, that works only up to AV, which can drop I think up to L40 eggs, maybe a little higher. Any zones past AV (Thunderhoof Hills and up) you get low level eggs, even though they have high level mobs. Same for Coo/Xaviera. Lousy eggs in all of them.
