If you are an individual, by which I mean you are not a commercial or non-profit enterprise, as specified by the regulations, then it is legal to own a serval in Texas provided you have a “certificate of registration” (i.e. a license) for the serval “issued by an animal registration agency”. In other words, you will need a certificate from the authorities. You don’t have to ‘own’ the serval. You simply have to do harbour it, control or have custody of it. Having custody of a serval does not necessarily mean that you own the animal. It means that you possess it at a certain time. Therefore, you will need a licence as specified if you are in possession of a serval. Certain listed enterprises e.g. zoos don’t need a certificate and you can read the list by visiting the law which is here. Scroll down to Sec. 822.102. APPLICABILITY OF SUBCHAPTER.

The certificate of registration is not transferable and is valid for one year after it was issued unless it has been revoked. The government agency which issued the licence may charge a fee to cover the costs of administering the process. The fee should not be more than $50 for each animal registered. And it should not be more than $500 for each person no matter how many animals are being registered.
You will need to fill in a special form as provided by the government authority. I won’t go over what you need to state in the application because the form will guide your through questions. When obtained the certificate will need to be displayed prominently at the place where the animal is kept. A copy of the certificate will need to be filed within 10 days with the Department of State Health Services. I expect this information to be on the application form.
The person who has ownership or custody of a serval in Texas will need to take out a liability insurance policy of not less than $100,000 “for each occurrence for liability for damages for destruction of or damage to property and death or bodily injury to a person caused by the dangerous wild animal i.e. the serval.
As a person with custody of a serval or servals you will need to allow local authority staff to enter their premises or home where the animal is kept to inspect the animal and the enclosure where the animal is kept if that is the case. They will also inspect “records relating to the animal to ensure compliance” with the law.

There are other specifications and conditions with which you have to comply which you can read about by clicking here (note: sometimes links to other websites break. If this has happened, I am sorry. Search for the statute by copying and pasting its title which is below). The link takes you to the actual law. That is the statute created by the politicians of Texas. The statute is called: HEALTH AND SAFETY CODE. TITLE 10. HEALTH AND SAFETY OF ANIMALS. CHAPTER 822. REGULATION OF ANIMALS.
My belief is that the question the title has been asked by an individual by which I mean a single person or couple, probably a citizen or citizens of the state of Texas. If that is correct then you will need a certificate of registration in order to have custody of, including ownership of, a serval which is described as a dangerous wild animal under the above-mentioned statute.
SOME MORE ON THE SERVAL: