Ignoring the lack of basic detail, and the incredibly tiny size of the suggestion, let's move on to problems with the general idea of dinosaurs:
- There's an obvious lore problem happening here. Minecraft isn't located on our world as we know it, considering the Zombies and Creepers, but it seems to be in a relatively medieval time at the earliest. Dinosaurs pose an obvious time problem, and it implies lore that Minecraft tries to avoid.
- Dinosaur names are usually long, scientific words. The animals and other creatures in the game, with a few exceptions, are normally single, one or two syllable words. Wolf, Horse, etc. are very easy for people to say and spell compared to Pterodactyl.
- Most animals and creatures are smaller than the player, or only slightly bigger. The only exception to this rule is the Ender Dragon. The reason for this is that it would be hard to give these creatures proper collision. Even the Ender Dragon doesn't actually land on blocks.
- The uses Dinosaurs would likely have are all covered by animals already in the game. Player and item transport, food and animal hides, etc. There are few roles that new animals can play besides diversity.
I get offtopic a lot, be warned. I also often mention a Minecraft clone called Minetest. The clone is nowhere near playable with it's default game in my opinion, but it's at least the start of a good open-source voxel rendering engine, and maybe one day it might be decently playable.
Like C1ff said, minecraft is set in medieval times in an unknown dimension thanks to the creeper and nether portal and stuff. However, it simply doesn't make sense that dinosaur would be added. Its like current minecraft is medieval time with some fantasy stuff added to make things fancier, but dinosaur is too ancient to be medieval.
Dinosaur are also generally scientific name. It would be difficult for a kid to pronounce Pterodactyl. When I was a kid, I used to pronounce the "P" in Pterodactyl, which cause the pronounciation to be wrong. The name are too scientific for all age game, or maybe too long. But both are definitely an issues.
There's fossil in the game. You think that dinosaur would stay alive after some kind of horrible meteor impact? And its hard for dinosaur to have any unique purpose. If anything, its better for the mob to be something unique instead of the so common dinosaur. I wouldn't mind something that resemble a dinosaur, but I don't want it to be named exactly like a dinosaur.
And what kind of dinosaur do you want? How is their rarity? Where do they spawn? And if so, what time? How much health? Are they aggresive or neutral? If neutral or aggresive, how much dmg? How to provoke them if neutral? Why would they be cool addition? What do they drop? Do they have mob relationship(like wolf with sheep and ocelot with creeper)? Do they have any other ability? What do they drop? What would their drop be used for?
That's some of the question that you should consider asking yourself before making a mob suggestion. If you can't answer those question, then chances are you are not ready to post yet. Even if your suggestion is disliked, then remember that if you make your thread detailed enough, then people will give you one point for effort. I like to read a detailed suggestion that is unbalanced rather than a suggestion with extreme unbalance and is very vague.
Dinosaurs as we know them wouldn't be a good addition to the game, but mobs that resemble dinosaurs (with shorter/easier to pronounce names eg. Crushler, Stampeder, Soarer?) would make for interesting new mobs. Perhaps, not to disrupt the medieval theme Minecraft has, they could reside in far off, hard to find biomes, making them 'mysterious'.