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- Iberolacerta aranica — Aran Rock Lizard
- Iberolacerta aurelioi — Aurelio's rock lizard
- Iberolacerta cyreni
- Iberolacerta martinezricai
- Icterus oberi — Montserrat Oriole
- Ideopsis hewitsonii — Hewitson's Small Tree-nymph
- Idiostatus middlekaufi — Middlekauf's Shieldback Katydid
- Idunella sketi
- Iglica kleinzellensis
- Ilyodon whitei
- Indirana brachytarsus
- Indirana diplosticta
- Indirana gundia
- Indirana leptodactyla
- Indirana phrynoderma
- Indolestes boninensis
- Indotestudo elongata — Elongated Tortoise, Pineapple Tortoise, Red-nosed Tortoise, Yellow Tortoise, Yellow-headed Tortoise
- Indotestudo forstenii — Celebes Tortoise, Forsten's Tortoise, Travancore Tortoise
- Indri indri — Indri
- Inflectarius magazinensis — Magazine mountain middle-toothed snail, Magazine mountain shagreen
- Ingolfiella longipes
- Insuetophrynus acarpicus
- Insulivitrina reticulata
- Io fluvialis — Spiny river snail
- Ischnura gemina — San Francisco Forktail
- Island Fox
- Isogomphodon oxyrhynchus — Daggernose shark
- Isolobodon portoricensis — Puerto Rican Hutia
- Isthmohyla angustilineata
- Isthmohyla calypsa
- Isthmohyla debilis
- Isthmohyla graceae
- Isthmohyla insolita
- Isthmohyla picadoi
- Isthmohyla rivularis
- Isthmohyla tica
- Ixalodectes flectocercus
- Ixalotriton niger
- Ixalotriton parva
- Ixos siquijorensis — Streak-breasted Bulbul
Read more about this topic: IUCN Red List Endangered Animal Species