It is even possible for them to generate even taller; while any single cactus will be 1-3 blocks tall they can generate on top of one another since the world generator uses the same conditions as when the player can place them (i.e. cactus is a valid block that they can be placed on) and it generates 10 patches of cactus (each with 10 attempts, 100 total, of a 1-3 block tall plant, averaging 2 blocks, so as many as 300 cactus blocks can potentially generate in a single chunk) per chunk at random altitudes so they can possibly be placed on top of existing cacti. Also, this is less likely to happen since 1.7 because their altitude range was increased from 0-127 to 0-255 in accordance with many other features due to terrain being able to exceed 127.
Also, while I have not noticed them in-game there are several cacti taller than 3 blocks visible in this map of the world I'm currently playing on (click to see full size; there are several that appear to be 4-5 blocks tall in an area near the center, surrounded by 3 cobblestone pillars in a triangular arrangement. All the others are fully grown, except at the very edges, so they are easy to spot elsewhere):
You can see a few in this map as well, one to the left of my base near the center (they appear shorter since I did not increase the height scale when rendering):
I'd say that it is actually not that rare to find cactus 4+ blocks high, certainly not as rare as a desert well (1 in 1000 chunks), which is actually slightly more common than a village or desert temple, which are 1/1024), though as noted above this is less likely to happen in more recent versions due to a doubling of the altitude range.
- Seen a group of twenty creepers in a cave in a world gen upon chunk-load for 1.12.2, seed 281292402360221982 at nearest village to spawn
-Seen a similar group of zombies once, lost world (in 1.6.x)
These have nothing to do with the world seed - the only hostile mobs that spawn with world generation are associated with structures like witch huts. It can seem like it since the game attempts to spawn hostile mobs once per tick so they are likely to spawn before the client can finish rendering the world. I'd also consider huge zombie hordes to be normal for 1.6.x; for example, I saw this outside of a village, and this outside of one of my bases (with a village inside. Neither were zombie sieges, which did not work in 1.6, nor have I fixed them (in fact, I disabled the broken code altogether to avoid wasting CPU cycles on failed attempts); their target range used to be up to 100 blocks (set to 40-100 per zombie) before Mojang severely nerfed them in 1.8, to 35-55 and based on regional difficulty so it only reaches 55 at the maximum (before it was independent so Easy and Hard were the same. 100 blocks covers 8 times the area of 35 blocks and 3.3 times the area of 55 blocks). For the same reason at least half of all mobs that I encounter are zombies even though they have the same spawn probability as creepers, skeletons, and spiders (an example of an extreme session).