One of the more infamous creatures to use its body as a cat call, the blue and green peacock (male of the peafowl species) uses enormous tail feathers to attract a mate. The exact function of the feathers is unknown, and their "attractiveness" may relate to their size, shape, patterns, or merely as a general health indicator. Plumes are famous for their "eye spots", and larger numbers of them often indicate a healthy male.
All species of deer have antlers which are grown and shed each year, including white-tailed deer, moose, and caribou. Antlers start off like velvet, and as mating season approaches they harden into bone. Female deer find larger antlers attractive, and males often use them in competition for mates, kind of like arm wrestling.
Anything named the "widowbird" must come with a high price for pairing up. Like many birds, males are brightly colored while females tend to have camouflage patterns on their feathers. What makes the widowbird unique is the 50cm (nearly two feet) long tailfeathers that weigh the males down and make them easy marks for predators. The only function they serve is attracting females.
Guppies perform a sort of mating dance to attract mates, the brightly colored males growing long tails to emphasize the way their bodies twist and turn. The tails can be full fan shaped, triangular like a pennant, or torn and tattered looking. Often the flaying of the tails comes from other guppies biting it!
Trading in suicidal colors or heavy body weight, the bowerbird chooses to construct elaborate nests to draw in mates. Bowerbirds have proven to be intelligent architects, using tricks like forced perspective (making the inside appear deeper than it actually is by using larger timbers near the door) to impress potential mates. Bowerbirds have also evolved enormous eyes to take in these unique nests.
The sage grouse is an odd animal where the male has the breasts in the family. Males use their chests as inflatable display pieces (almost like a puffer-fish) as a part of its mating dance, taking on the look of a rather odd yet comfortable looking parka. Possibly due to how silly it can look to have a whole troupe of males dancing, sage grouse tend to perform these dances with some cover from their favored sagebrush. Perhaps they're just shy to perform out in the open.
Of all those bird ornamentations, the frigatebird probably has the most ornate. Over a span of about 20 minutes, the males can inflate their throats into gigantic, red, heart-shaped balloons. The femal will then go to the male with the largest throat sac, and while procreating, the male will cover her eyes with his wings so she won't be distracting by other giant red throats sacs.
Red-Sided Garter Snake
These small snakes have possibly the most bizarre mating ritual of any animal. When a female emerges from hibernation, she releases a pheromone that attracts hundreds of males to surround her in a gigantic mating ball. And if having hundreds of males surround one female isn't enough, each male has two penises! The bizarre ritual is such a sight, that it's become a tourist attraction in Manitoba, Canada.
When it comes to clowinfish breeding, size certainly does matter. These fish actually live in groups with only one breeding couple, and that couple is chosen: the largest clownfish is the female and the second largest is the male. Did I mention that clownfish are able to change genders? If the group's breeding female meets her end, then the breeding male will switch genders and become the female, and the next largest clownfish will be promoted to the breeding male!
Galapagos Giant Tortoise
Galapagos Giant Tortoises really have to stick their neck out to get a girl, literally. The males will stretch out their necks to determine who gets to mate, and only the tortoise with the longest neck will get to mount a female for their hours-long (and surprisingly noisy) coitus. The shorter necked males will either resort to humping rocks or each other.
Cupid's love arrow may be just a myth, but not so for snails. These shelled slugs are actually hermaphrodites, but they don't self-fertalize; instead they fire "love darts" at one another which increase the amount of sperm that can be stored in the snail's uterus.
Bedbugs may spend all their time in the bed, but they clearly know nothing about romance. Males won't even bother to find the females' sex organs! Instead, they use their sharp sexual organs to impale a female and deposit their sperm. Scientists have descripribed this horrific event best, calling it "traumatic insemination."
The male argonaut, also know as the paper nautilus, will actually detach his penis to attract a female. Maybe the male is intimidated by the much larger female (females can be up to 18 times larger!), but instead of swimming over himself, he detaches a tentacle filled with his sperm and will stay back as just the tentacle swims up to the female.