CompInitialAbility makes an animal always spawn with an initial ability present. Bear in mind that certain abilities will only work if the animal is also draftable, so test throroughly

    //A comp class that makes animals always spawn with an initial ability

    public AbilityDef initialAbility;

It is a comp class, so you just add it in XML in the <comps> tag. For example, this allows the Thrumborat in Vanilla Genetics Expanded to always have the burrow ability

<comps>
	<li Class="AnimalBehaviours.CompProperties_InitialAbility">
		<initialAbility>GR_Burrow</initialAbility>
	</li>
</comps>