Just tell the player that you are not comfortable with this for the first campaign you are running for the group. It seems that this one player is already causing you discomfort and sucking up your time trying to find ways to accommodate his wishes. Putting together a new campaign for a new group should be exciting and fun, not something that creates anxiety. Rip off the bandaid and just say no.
For me the issue isn't whether this is appropriate for D&D. If it fit into the kind of campaign I and most of my players wanted to play, I would find a way to make it work. But if it doesn't, I have no issues saying "sorry, but that doesn't fit in with our current campaign." If he is cool with that, I'll work with him to come up with something that works for him and the campaign. If he pushes back, I'm comfortable saying that this probably isn't the game he is looking for and moving on.
If the rejected player is friends with the other players and his being rejected from the game leads to push back from the other players, then you need to have a discussion on what kind of game they want to run. If they want to play in a campaign where a dinosaur character fits in, then you have to determine if you can rejigger the planned campaign to make it work or start from scratch with a new campaign. If you feel that you are unable to, or just don't want to, do that, you should suggest one of the other players DM.
If none of them want to because they are new to the game, if it were ME, I'm not going to be guilted into running a game that I'm not going to enjoy running. If I have to put a lot more work into prepping and running a campaign to accommodate the players' preferences, when I've already prepared a campaign that I thought we had all agreed on, I'm not going to enjoy all that extra work. Maybe when I was younger, but I'm too busy at this stage in my life. At this point, I would probably politely exit from the group. I would apologize and just state that I'm too busy. No gaming is better than bad gaming. But more than that, I can find good gaming online. I'm not going to get myself into running a game I don't enjoy, just to play in person.
In my over 10 years since getting back into gaming, I've never had this problem. With my first campaign, after getting back into TTRPGs with 5e, I had two old high school friends who were up for the game and then I posted a detailed call for players in a couple of game-related meetups. I wrote up a description of the campaign world and detailed character restrictions. I didn't have anyone contact me who wanted something different. For those who did reply, I had some back and forth over e-mail.
In every campaign since, I generally give 3-5 options for the next campaign and discuss with my players a few months before the current campaign is expected to wrap up. After we come to a consensus, I start preparing that campaign.