Pampas deer (Ozotoceros bezoarticus), a cervid counterpart to a combination of reedbuck (Redunca) and oribi (Ourebia)

https://naturerules1.fandom.com/wiki/Pampas_Deer?file=44be4bb6adc104c8b09adc6442325288.jpg

https://www.flickriver.com/photos/tags/veadocampeiro/interesting/

https://www.zoochat.com/community/media/2013-bioparque-mbopicua.231505/

https://www.flickriver.com/photos/tags/ozotocerosbezoarticus/interesting/

@paradoxornithidae

I agree that, of all the South American deer other than Odocoileus virginianus itself (which has penetrated quite deeply into this continent), Ozotoceros bezoarticus is the one most resembling O. virginianus.

The main differences seem to be in body size and the proportional size of the tail.

At about 25 kg average body mass for adult females, O. bezoarticus is smaller than the smallest subspecies of O. virginianus (about 30 kg). It is also smaller than any species of reedbuck (Redunca fulvorufula, about 30 kg).

So, diminution is an important aspect of the evolution of O. bezoarticus, relative to both ancestral Odocoileus and ecologically comparable Redunca.

The proportional diminution of the tail in O. bezoarticus is understandable in view of the plasticity of the size of the tail in Odocoileus. This has brought the size of the tail in O. bezoarticus towards that in Redunca redunca. However, a considerable difference remains, in not only in the proportional size of the tail, but also its colouration and activation as a caudal flag.

Redunca redunca: https://upload.wikimedia.org/wikipedia/commons/b/b5/Bohor_Reedbuck%2C_female%2C_Serengeti.jpg

Ozotoceros bezoarticus:
https://www.flickr.com/photos/janharteman/8058927300/

The tail and hindquarters of O. b. leucogaster resemble those of Ourebia montana (body mass about 15 kg) as much as those of Redunca:
https://psnp.info/p4td_/?p=15987

They also resemble those of some individuals of Odocoileus hemionus columbianus:
https://www.alamy.com/stock-image-cautious-black-tailed-deer-looking-back-167296028.html?imageid=CF489FF2-F8E6-4478-862A-B46B1E801B70&p=259859&pn=2&searchId=b73df524765636058fee1000c4f1963d&searchtype=0

However, most individuals of the latter have the tail longer than in O. bezoarticus:
https://www.alamy.com/columbian-black-tailed-deer-family-grazing-rear-views-image417742085.html

https://www.reddit.com/r/Awwducational/comments/cfxxyb/pampas_deer_are_the_most_polymorphic_mammals_and/

https://www.alamy.com/stock-photo-deer-capreolus-capreolus-looking-back-field-scenery-national-park-132451106.html?imageid=81820564-BC84-4006-BDC1-B52C1E351BB9&p=381589&pn=1&searchId=405d3db25091a0baad8b5b010a1268c1&searchtype=0

https://www.alamy.com/close-up-of-young-black-tailed-deer-yosemite-national-park-california-image225472388.html?imageid=5C8DC24B-939D-46C7-9ACF-FB2A674EF270&p=1313754&pn=1&searchId=405d3db25091a0baad8b5b010a1268c1&searchtype=0

Shift
One of the best illustrations of juvenile in Ourebia montana cottoni/masakensis:
https://psnp.info/p4td_/?p=15987

Axis axis

https://www.alamy.com/stock-photo-a-herd-of-spotted-deer-yala-national-park-sri-lanka-34540878.html?imageid=FDC465C9-CE75-4DE4-9890-864E7B5442A8&p=78310&pn=1&searchId=405d3db25091a0baad8b5b010a1268c1&searchtype=0

Axis porcinus

https://www.alamy.com/kaziranga-national-park-assam-northeastern-india-image337728500.html?imageid=967413C7-783D-4459-A810-DDB26BD0C955&p=132086&pn=1&searchId=405d3db25091a0baad8b5b010a1268c1&searchtype=0

Cervus nippon
https://www.alamy.com/male-deer-looking-back-in-the-winter-forest-animal-in-natural-habitat-wildlife-scene-image458096157.html?imageid=8B28F081-4710-46D3-991D-7391B2FFAB42&p=280977&pn=1&searchId=405d3db25091a0baad8b5b010a1268c1&searchtype=0

Odocoileus hemionus
https://www.alamy.com/a-mule-deer-buck-in-velvet-antlers-looking-back-from-a-forest-in-northern-montana-image179802650.html?imageid=504D9C0F-4CA0-4D17-97B5-F7851E2E8504&p=92827&pn=1&searchId=405d3db25091a0baad8b5b010a1268c1&searchtype=0

@paradoxornithidae

The photo-pair you linked shows the almost unbelievable evolutionary plasticity of the caudal flag/hindquarters bleeze in Odocoileus.

Odocoileus hemionus is thought to have evolved recently from Odocoileus virginianus, and the two species remain able to interbreed. However, the pattern on the hindquarters is almost unrecognisably different, in conjunction with the tail no longer being flagged in O. hemionus hemionus.

One reason why, in cervids, the tail becomes small, is reduction of surface area in cold climates. This seems to explain the relatively small tail in O. hemionus hemionus to some extent.

However, this explanation would not apply to O. bezoarticus.

So, the diminution of the tail in O. bezoarticus (25 kg), relative to both ancestral Odocoileus and ecologically analogous Redunca, is one of the puzzling aspects of the pampas deer. It suggests convergence with Ourebia montana (15 kg) as much as Redunca redunca (35 kg).

So, a fair 'working model' for O. bezoarticus seems to be that it is the ecological counterpart for a combination of Redunca redunca (https://www.shutterstock.com/image-photo/oribi-ourebia-ourebi-kenya-afirca-1922206295) and Ourebia montana (https://www.natureinstock.com/search/preview/oribi-ourebia-ourebi-murchison-falls-national-park-uganda/0_00127717.html).

This would make sense, given how widespread these two spp. of bovids are in African savannas. However, the sexual dimorphism in the growth of head-adornments remains different in cervid vs bovid.

Coming back to your question re the tail: O. bezoarticus would seem to have a diminutive tail for the same reasons that Ourebia, Raphicerus, and Madoqua have diminutive tails. What could that reason be?

https://academic.oup.com/jmammal/article/83/4/1127/2373226?login=false

https://bioone.org/journals/wildlife-biology/volume-14/issue-3/0909-6396(2008)14%5B350%3ARIASHO%5D2.0.CO%3B2/Reproduction-in-a-semi-captive-herd-of-pampas-deer-Ozotoceros/10.2981/0909-6396(2008)14[350:RIASHO]2.0.CO;2.full

https://onlinelibrary.wiley.com/doi/pdf/10.2981/0909-6396%282008%2914%5B350%3ARIASHO%5D2.0.CO%3B2

excellent images, such subtle, but distinct, differences.