Mont. Code Ann. § 81-4-511

Following a roundup held pursuant to this part, the department of revenue shall immediately assess all horses that have been taken up in a roundup and that may be sold or reclaimed before sale and not already assessed for the current year. The department of revenue shall transmit to the county treasurer a copy of each assessment made. Any horses that have escaped the assessment mentioned in 81-4-501 must be assessed as provided for in 15-8-306.