Legal Question in Family Law in New Mexico

If he lived with the 8 year old for the first two years of her life and has openly held her out as his own, then he may be the "presumed" father of that child under current law - NMSA 40-11A-5. Under the law prior to 2010, a man may be the presumed father if " the child is under the age of majority, he openly holds out the child as his natural child and has established a personal, financial or custodial relationship with the child." NMSA 40-11-5, repealed Jan 1, 2010 (note that there is not a requirement that he have lived with the child for the first two years, as in the current law). That could apply to both children as you say you were married before that law was repealed and the new law came into effect. He could possibly successfully claim to be the presumed father under either of those. It is a rebuttable presumption, however, meaning you could rebut it in various ways including identifying the biological father and using DNA testing.

As the presumed father he would have to go to court to get an adjudication of parentage and then he could have joint custody, also have to pay child support, etc., as any father would in the event of a divorce. The presumption of paternity goes both ways: it works for him if he's claiming "rights" to the child, or it works for you if you're claiming child support. Likewise the ability to rebut the presumption works both ways.

Re: the horse park - you can agree to keep the horse park his separate property but you don't have to. If he is buying it with community funds or mortgaging it then it will be community property and community debt unless you specifically agree that it is separate property. Don't let him buy separate property and have a mortgage on it where the loan documents do not specify that the loan is his separate debt. Community funds means any money he earned while you were married, unless it was income from separate property that he owned prior to your marriage.

Both of these issues (paternity and property) can get pretty complicated and you would be well advised to take all the documents to an attorney so he can analyze them based on the facts of your situation.