For players who have never encountered Zomborg before, they would have no idea just how much he would be buffed or how fast he would evolve, and thus creates a problem when planning ahead. Even players who have faced him before would have a hard time remembering.
I suggest having some sort of timer on the card, counting down the number of turns to the turn he transforms into a Dragon, and also how much he would be buffed on the next turn. For instance, he would start the game with 'Turns left to full evolution: 5' and 'Next evolution: +1/+1'. Subsequent evolutions would read:
'+1/+1, gains Infect',
'+3/+2, Pay 2: Deal 1 damage to all deployed characters and gain Flying',
'+2/+1, ability damage and cost x2',
'+9/+1, gains Unstoppable and Flying, loses ability, creates 6 Endless Horde in Grave when killed'.