Well, it didn't work 100% because of a missing texture, but it does work. If you can make a pack texture, I think this is good for the Thrumbo;


<Operation Class="PatchOperationAdd"> <!-- Pack Thrumbo -->
<xpath>/Defs/ThingDef[defName = "Thrumbo"]/race</xpath>
<value>
<packAnimal>true</packAnimal>
</value>
</Operation>

<Operation Class="PatchOperationReplace"> <!-- Upping bodysize to make them close to their class and description -->
<xpath>/Defs/ThingDef[defName = "Thrumbo"]/race/baseBodySize</xpath>
<value>
<baseBodySize>12.0</baseBodySize>
</value>
</Operation>

Alternatively of course you can choose a different body size, but in no way should they be smaller than an elephant based on the rarity and description provided. I'd like a graphic that actually has them hauling something behind them rather than a pack on their back, but anyway. :P