Hi @Megan1336
This is a very difficult area for Hosts and I would tread very, very cautiously. I would carefully read through Airbnb's policies regarding ESAs and Service Animals:
Service Animals & ESAs
https://www.airbnb.com/help/article/1869
I find it highly unlikely the guest has two ESAs. That being the case, you can advise them of Airbnbs policies regarding ESAs in a message and outline your policies for them. I would give them the above link. NEVER refer to the ESAs as pets. You can charge a pet fee for ESAs, but obviously the guest is trying to get around that by not indicating "pets" on the reservation. BTW, I think your pet fee is a bit low. You can say that you fully comply with all Airbnb policies regarding ESAs and that Airbnb's policies do allow you to charge a fee for ESAs and the fee will be $XX.XX (don't call it a "Pet Fee").
Unfortunately, your Addl Rules only refer to "Pets" not ESAs. I would change that immediately, so this issue doesn't come up again. Technically, an ESA is not a pet, so this is where they are trying to work the system. Since you didn't state BOTH pets and ESAs incur a fee, not sure you are going to be able to add a fee now. You can try, but my guess is this guest has done this before to other hosts and knows how to work the system.
You could try sending your ESA Policy to the guest regarding breed size, must be on a leash, can't be left alone at the listing etc. but the guest could simply state those rules don't apply to them as it's for "Pets" not ESAs.
You may just have to chalk this up to experience and hope the "ESAs" are not Great Danes that will wreak havoc on your place and cause alot of damage. If you try to force the issue, you are almost guaranteed to get a bad review that could really hurt you as a new host. I would update your Addl Rules to include ESAs in the wording.
If there is damage done, you can try requesting reimbursement through Aircover using a Resolution Request. However, it must be above the "normal" cleaning your place would need since you do allow pets. Whatever you do, don't let on to the guest that you will be charging them for damages. That is guaranteed to get your a bad review, and possibly they will complain to Airbnb that you discriminated against them due to their ESAs. This could lead to Airbnb suspending your listing while they "investigate." Let them post their review, start the claim right away but don't post the claim unitl the last minute (14days) and then also post your honest review at the last minute, right before the 14day cutoff.